diff --git a/core/assets/vendor/ckeditor5/build/paste-from-office.js b/core/assets/vendor/ckeditor5/build/paste-from-office.js
new file mode 100644
index 0000000000..00828ee5ff
--- /dev/null
+++ b/core/assets/vendor/ckeditor5/build/paste-from-office.js
@@ -0,0 +1,5 @@
+/*!
+ * @license Copyright (c) 2003-2021, CKSource - Frederico Knabben. All rights reserved.
+ * For licensing, see LICENSE.md.
+ */
+window.CKEditor5=window.CKEditor5||{},window.CKEditor5.pasteFromOffice=function(e){var t={};function n(r){if(t[r])return t[r].exports;var i=t[r]={i:r,l:!1,exports:{}};return e[r].call(i.exports,i,i.exports,n),i.l=!0,i.exports}return n.m=e,n.c=t,n.d=function(e,t,r){n.o(e,t)||Object.defineProperty(e,t,{enumerable:!0,get:r})},n.r=function(e){"undefined"!=typeof Symbol&&Symbol.toStringTag&&Object.defineProperty(e,Symbol.toStringTag,{value:"Module"}),Object.defineProperty(e,"__esModule",{value:!0})},n.t=function(e,t){if(1&t&&(e=n(e)),8&t)return e;if(4&t&&"object"==typeof e&&e&&e.__esModule)return e;var r=Object.create(null);if(n.r(r),Object.defineProperty(r,"default",{enumerable:!0,value:e}),2&t&&"string"!=typeof e)for(var i in e)n.d(r,i,function(t){return e[t]}.bind(null,i));return r},n.n=function(e){var t=e&&e.__esModule?function(){return e.default}:function(){return e};return n.d(t,"a",t),t},n.o=function(e,t){return Object.prototype.hasOwnProperty.call(e,t)},n.p="",n(n.s=4)}([function(e,t,n){e.exports=n(1)("./src/engine.js")},function(e,t){e.exports=CKEditor5.dll},function(e,t,n){e.exports=n(1)("./src/core.js")},function(e,t,n){e.exports=n(1)("./src/clipboard.js")},function(e,t,n){"use strict";n.r(t),n.d(t,"PasteFromOffice",(function(){return b}));var r=n(2),i=n(3),o=n(0);function s(e,t){if(!e.childCount)return;const n=new o.UpcastWriter(e.document),r=function(e,t){const n=t.createRangeIn(e),r=new o.Matcher({name:/^p|h\d+$/,styles:{"mso-list":/.*/}}),i=[];for(const e of n)if("elementStart"===e.type&&r.match(e.item)){const t=a(e.item);i.push({element:e.item,id:t.id,order:t.order,indent:t.indent})}return i}(e,n);if(!r.length)return;let i=null,s=1;r.forEach((e,a)=>{const u=function(e,t){if(!e)return!0;if(e.id!==t.id)return t.indent-e.indent!=1;const n=t.element.previousSibling;if(!n)return!0;return r=n,!(r.is("element","ol")||r.is("element","ul"));var r}(r[a-1],e),f=u?null:r[a-1],d=(m=e,(p=f)?m.indent-p.indent:m.indent-1);var p,m;if(u&&(i=null,s=1),!i||0!==d){const r=function(e,t){const n=new RegExp(`@list l${e.id}:level${e.indent}\\s*({[^}]*)`,"gi"),r=/mso-level-number-format:([^;]{0,100});/gi,i=n.exec(t);let o="decimal",s="ol";if(i&&i[1]){const t=r.exec(i[1]);if(t&&t[1]&&(o=t[1].trim(),s="bullet"!==o&&"image"!==o?"ol":"ul"),"bullet"===o){const t=function(e){const t=function(e){if(e.getChild(0).is("$text"))return null;for(const t of e.getChildren()){if(!t.is("element","span"))continue;const e=t.getChild(0);return e.is("$text")?e:e.getChild(0)}}(e);if(!t)return null;const n=t._data;if("o"===n)return"circle";if("·"===n)return"disc";if("§"===n)return"square";return null}(e.element);t&&(o=t)}}return{type:s,style:c(o)}}(e,t);if(i){if(e.indent>s){const e=i.getChild(i.childCount-1),t=e.getChild(e.childCount-1);i=l(r,t,n),s+=1}else if(e.indent<s){const t=s-e.indent;i=function(e,t){const n=e.getAncestors({parentFirst:!0});let r=null,i=0;for(const e of n)if("ul"!==e.name&&"ol"!==e.name||i++,i===t){r=e;break}return r}(i,t),s=parseInt(e.indent)}}else i=l(r,e.element,n);e.indent<=s&&(i.is("element",r.type)||(i=n.rename(r.type,i)))}const g=function(e,t){return function(e,t){const n=new o.Matcher({name:"span",styles:{"mso-list":"Ignore"}}),r=t.createRangeIn(e);for(const e of r)"elementStart"===e.type&&n.match(e.item)&&t.remove(e.item)}(e,t),t.rename("li",e)}(e.element,n);n.appendChild(g,i)})}function c(e){switch(e){case"arabic-leading-zero":return"decimal-leading-zero";case"alpha-upper":return"upper-alpha";case"alpha-lower":return"lower-alpha";case"roman-upper":return"upper-roman";case"roman-lower":return"lower-roman";case"circle":case"disc":case"square":return e;default:return null}}function l(e,t,n){const r=t.parent,i=n.createElement(e.type),o=r.getChildIndex(t)+1;return n.insertChild(o,i,r),e.style&&n.setStyle("list-style-type",e.style,i),i}function a(e){const t={},n=e.getStyle("mso-list");if(n){const e=n.match(/(^|\s{1,100})l(\d+)/i),r=n.match(/\s{0,100}lfo(\d+)/i),i=n.match(/\s{0,100}level(\d+)/i);e&&r&&i&&(t.id=e[2],t.order=r[1],t.indent=i[1])}return t}const u=/id=("|')docs-internal-guid-[-0-9a-f]+("|')/i;class f{constructor(e){this.document=e}isActive(e){return u.test(e)}execute(e){const t=new o.UpcastWriter(this.document),{body:n}=e._parsedData;!function(e,t){for(const n of e.getChildren())if(n.is("element","b")&&"normal"===n.getStyle("font-weight")){const r=e.getChildIndex(n);t.remove(n),t.insertChild(r,n.getChildren(),e)}}(n,t),function(e,t){for(const n of t.createRangeIn(e)){const e=n.item;if(e.is("element","li")){const n=e.getChild(0);n&&n.is("element","p")&&t.unwrapElement(n)}}}(n,t),e.content=n}}function d(e,t){if(!e.childCount)return;const n=new o.UpcastWriter;!function(e,t,n){const r=n.createRangeIn(t),i=new o.Matcher({name:"img"}),s=[];for(const t of r)if(i.match(t.item)){const n=t.item,r=n.getAttribute("v:shapes")?n.getAttribute("v:shapes").split(" "):[];r.length&&r.every(t=>e.indexOf(t)>-1)?s.push(n):n.getAttribute("src")||s.push(n)}for(const e of s)n.remove(e)}(function(e,t){const n=t.createRangeIn(e),r=new o.Matcher({name:/v:(.+)/}),i=[];for(const e of n){const t=e.item,n=t.previousSibling&&t.previousSibling.name||null;r.match(t)&&t.getAttribute("o:gfxdata")&&"v:shapetype"!==n&&i.push(e.item.getAttribute("id"))}return i}(e,n),e,n),function(e,t){const n=t.createRangeIn(e),r=new o.Matcher({name:/v:(.+)/}),i=[];for(const e of n)r.match(e.item)&&i.push(e.item);for(const e of i)t.remove(e)}(e,n);const r=function(e,t){const n=t.createRangeIn(e),r=new o.Matcher({name:"img"}),i=[];for(const e of n)r.match(e.item)&&e.item.getAttribute("src").startsWith("file://")&&i.push(e.item);return i}(e,n);r.length&&function(e,t,n){if(e.length===t.length)for(let i=0;i<e.length;i++){const o=`data:${t[i].type};base64,${r=t[i].hex,btoa(r.match(/\w{2}/g).map(e=>String.fromCharCode(parseInt(e,16))).join(""))}`;n.setAttribute("src",o,e[i])}var r}(r,function(e){if(!e)return[];const t=/{\\pict[\s\S]+?\\bliptag-?\d+(\\blipupi-?\d+)?({\\\*\\blipuid\s?[\da-fA-F]+)?[\s}]*?/,n=new RegExp("(?:("+t.source+"))([\\da-fA-F\\s]+)\\}","g"),r=e.match(n),i=[];if(r)for(const e of r){let n=!1;e.includes("\\pngblip")?n="image/png":e.includes("\\jpegblip")&&(n="image/jpeg"),n&&i.push({hex:e.replace(t,"").replace(/[^\da-fA-F]/g,""),type:n})}return i}(t),n)}const p=/<meta\s*name="?generator"?\s*content="?microsoft\s*word\s*\d+"?\/?>/i,m=/xmlns:o="urn:schemas-microsoft-com/i;class g{constructor(e){this.document=e}isActive(e){return p.test(e)||m.test(e)}execute(e){const{body:t,stylesString:n}=e._parsedData;s(t,n),d(t,e.dataTransfer.getData("text/rtf")),e.content=t}}function h(e){return e.replace(/<span(?: class="Apple-converted-space"|)>(\s+)<\/span>/g,(e,t)=>1===t.length?" ":Array(t.length+1).join("  ").substr(0,t.length))}function y(e,t){const n=new DOMParser,r=function(e){return h(h(e)).replace(/(<span\s+style=['"]mso-spacerun:yes['"]>[^\S\r\n]*?)[\r\n]+([^\S\r\n]*<\/span>)/g,"$1$2").replace(/<span\s+style=['"]mso-spacerun:yes['"]><\/span>/g,"").replace(/ <\//g," </").replace(/ <o:p><\/o:p>/g," <o:p></o:p>").replace(/<o:p>(&nbsp;|\u00A0)<\/o:p>/g,"").replace(/>([^\S\r\n]*[\r\n]\s*)</g,"><")}(function(e){const t=e.indexOf("</body>");if(t<0)return e;const n=e.indexOf("</html>",t+"</body>".length);return e.substring(0,t+"</body>".length)+(n>=0?e.substring(n):"")}(e=e.replace(/<!--\[if gte vml 1]>/g,""))),i=n.parseFromString(r,"text/html");!function(e){e.querySelectorAll("span[style*=spacerun]").forEach(e=>{const t=e.innerText.length||0;e.innerHTML=Array(t+1).join("  ").substr(0,t)})}(i);const s=i.body.innerHTML,c=function(e,t){const n=new o.ViewDocument(t),r=new o.DomConverter(n,{blockFillerMode:"nbsp"}),i=e.createDocumentFragment(),s=e.body.childNodes;for(;s.length>0;)i.appendChild(s[0]);return r.domToView(i,{skipComments:!0})}(i,t),l=function(e){const t=[],n=[],r=Array.from(e.getElementsByTagName("style"));for(const e of r)e.sheet&&e.sheet.cssRules&&e.sheet.cssRules.length&&(t.push(e.sheet),n.push(e.innerHTML));return{styles:t,stylesString:n.join(" ")}}(i);return{body:c,bodyString:s,styles:l.styles,stylesString:l.stylesString}}class b extends r.Plugin{static get pluginName(){return"PasteFromOffice"}static get requires(){return[i.ClipboardPipeline]}init(){const e=this.editor,t=e.editing.view.document,n=[];n.push(new g(t)),n.push(new f(t)),e.plugins.get("ClipboardPipeline").on("inputTransformation",(e,r)=>{if(r._isTransformedWithPasteFromOffice)return;const i=r.dataTransfer.getData("text/html"),o=n.find(e=>e.isActive(i));o&&(r._parsedData=y(i,t.stylesProcessor),o.execute(r),r._isTransformedWithPasteFromOffice=!0)},{priority:"high"})}}}]);
\ No newline at end of file
diff --git a/core/assets/vendor/ckeditor5/drupal/src/drupalEmphasis/src/drupalemphasis.js b/core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalEmphasis/src/drupalemphasis.js
similarity index 100%
rename from core/assets/vendor/ckeditor5/drupal/src/drupalEmphasis/src/drupalemphasis.js
rename to core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalEmphasis/src/drupalemphasis.js
diff --git a/core/assets/vendor/ckeditor5/drupal/src/drupalEmphasis/src/drupalemphasisediting.js b/core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalEmphasis/src/drupalemphasisediting.js
similarity index 100%
rename from core/assets/vendor/ckeditor5/drupal/src/drupalEmphasis/src/drupalemphasisediting.js
rename to core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalEmphasis/src/drupalemphasisediting.js
diff --git a/core/assets/vendor/ckeditor5/drupal/src/drupalEmphasis/src/index.js b/core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalEmphasis/src/index.js
similarity index 100%
rename from core/assets/vendor/ckeditor5/drupal/src/drupalEmphasis/src/index.js
rename to core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalEmphasis/src/index.js
diff --git a/core/assets/vendor/ckeditor5/drupal/src/drupalImage/README.md b/core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalImage/README.md
similarity index 100%
rename from core/assets/vendor/ckeditor5/drupal/src/drupalImage/README.md
rename to core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalImage/README.md
diff --git a/core/assets/vendor/ckeditor5/drupal/src/drupalImage/src/drupalimage.js b/core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalImage/src/drupalimage.js
similarity index 100%
rename from core/assets/vendor/ckeditor5/drupal/src/drupalImage/src/drupalimage.js
rename to core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalImage/src/drupalimage.js
diff --git a/core/assets/vendor/ckeditor5/drupal/src/drupalImage/src/drupalimageediting.js b/core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalImage/src/drupalimageediting.js
similarity index 100%
rename from core/assets/vendor/ckeditor5/drupal/src/drupalImage/src/drupalimageediting.js
rename to core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalImage/src/drupalimageediting.js
diff --git a/core/assets/vendor/ckeditor5/drupal/src/drupalImage/src/imageupload/drupalfilerepository.js b/core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalImage/src/imageupload/drupalfilerepository.js
similarity index 100%
rename from core/assets/vendor/ckeditor5/drupal/src/drupalImage/src/imageupload/drupalfilerepository.js
rename to core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalImage/src/imageupload/drupalfilerepository.js
diff --git a/core/assets/vendor/ckeditor5/drupal/src/drupalImage/src/imageupload/drupalimageupload.js b/core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalImage/src/imageupload/drupalimageupload.js
similarity index 100%
rename from core/assets/vendor/ckeditor5/drupal/src/drupalImage/src/imageupload/drupalimageupload.js
rename to core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalImage/src/imageupload/drupalimageupload.js
diff --git a/core/assets/vendor/ckeditor5/drupal/src/drupalImage/src/imageupload/drupalimageuploadadapter.js b/core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalImage/src/imageupload/drupalimageuploadadapter.js
similarity index 100%
rename from core/assets/vendor/ckeditor5/drupal/src/drupalImage/src/imageupload/drupalimageuploadadapter.js
rename to core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalImage/src/imageupload/drupalimageuploadadapter.js
diff --git a/core/assets/vendor/ckeditor5/drupal/src/drupalImage/src/imageupload/drupalimageuploadediting.js b/core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalImage/src/imageupload/drupalimageuploadediting.js
similarity index 100%
rename from core/assets/vendor/ckeditor5/drupal/src/drupalImage/src/imageupload/drupalimageuploadediting.js
rename to core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalImage/src/imageupload/drupalimageuploadediting.js
diff --git a/core/assets/vendor/ckeditor5/drupal/src/drupalImage/src/index.js b/core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalImage/src/index.js
similarity index 100%
rename from core/assets/vendor/ckeditor5/drupal/src/drupalImage/src/index.js
rename to core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalImage/src/index.js
diff --git a/core/assets/vendor/ckeditor5/drupal/src/drupalMedia/README.md b/core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalMedia/README.md
similarity index 100%
rename from core/assets/vendor/ckeditor5/drupal/src/drupalMedia/README.md
rename to core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalMedia/README.md
diff --git a/core/assets/vendor/ckeditor5/drupal/src/drupalMedia/src/drupalmedia.js b/core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalMedia/src/drupalmedia.js
similarity index 100%
rename from core/assets/vendor/ckeditor5/drupal/src/drupalMedia/src/drupalmedia.js
rename to core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalMedia/src/drupalmedia.js
diff --git a/core/assets/vendor/ckeditor5/drupal/src/drupalMedia/src/drupalmediaediting.js b/core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalMedia/src/drupalmediaediting.js
similarity index 100%
rename from core/assets/vendor/ckeditor5/drupal/src/drupalMedia/src/drupalmediaediting.js
rename to core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalMedia/src/drupalmediaediting.js
diff --git a/core/assets/vendor/ckeditor5/drupal/src/drupalMedia/src/drupalmediatoolbar.js b/core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalMedia/src/drupalmediatoolbar.js
similarity index 100%
rename from core/assets/vendor/ckeditor5/drupal/src/drupalMedia/src/drupalmediatoolbar.js
rename to core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalMedia/src/drupalmediatoolbar.js
diff --git a/core/assets/vendor/ckeditor5/drupal/src/drupalMedia/src/drupalmediaui.js b/core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalMedia/src/drupalmediaui.js
similarity index 97%
rename from core/assets/vendor/ckeditor5/drupal/src/drupalMedia/src/drupalmediaui.js
rename to core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalMedia/src/drupalmediaui.js
index 75cbbd6f77..429256ee32 100644
--- a/core/assets/vendor/ckeditor5/drupal/src/drupalMedia/src/drupalmediaui.js
+++ b/core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalMedia/src/drupalmediaui.js
@@ -1,3 +1,5 @@
+// cspell:ignore medialibrary
+
 import { Plugin } from 'ckeditor5/src/core';
 import { ButtonView } from 'ckeditor5/src/ui';
 // cspell:ignore medialibrary
diff --git a/core/assets/vendor/ckeditor5/drupal/src/drupalMedia/src/index.js b/core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalMedia/src/index.js
similarity index 91%
rename from core/assets/vendor/ckeditor5/drupal/src/drupalMedia/src/index.js
rename to core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalMedia/src/index.js
index f52e6bc9ca..ba3c4ea0cd 100644
--- a/core/assets/vendor/ckeditor5/drupal/src/drupalMedia/src/index.js
+++ b/core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalMedia/src/index.js
@@ -1,3 +1,5 @@
+// cspell:ignore mediaimagetextalternative
+
 import DrupalMedia from './drupalmedia';
 // cspell:ignore mediaimagetextalternative
 import MediaImageTextAlternative from './mediaimagetextalternative';
diff --git a/core/assets/vendor/ckeditor5/drupal/src/drupalMedia/src/insertdrupalmedia.js b/core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalMedia/src/insertdrupalmedia.js
similarity index 100%
rename from core/assets/vendor/ckeditor5/drupal/src/drupalMedia/src/insertdrupalmedia.js
rename to core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalMedia/src/insertdrupalmedia.js
diff --git a/core/assets/vendor/ckeditor5/drupal/src/drupalMedia/src/mediaimagetextalternative.js b/core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalMedia/src/mediaimagetextalternative.js
similarity index 100%
rename from core/assets/vendor/ckeditor5/drupal/src/drupalMedia/src/mediaimagetextalternative.js
rename to core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalMedia/src/mediaimagetextalternative.js
diff --git a/core/assets/vendor/ckeditor5/drupal/src/drupalMedia/src/mediaimagetextalternative/mediaimagetextalternativecommand.js b/core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalMedia/src/mediaimagetextalternative/mediaimagetextalternativecommand.js
similarity index 100%
rename from core/assets/vendor/ckeditor5/drupal/src/drupalMedia/src/mediaimagetextalternative/mediaimagetextalternativecommand.js
rename to core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalMedia/src/mediaimagetextalternative/mediaimagetextalternativecommand.js
diff --git a/core/assets/vendor/ckeditor5/drupal/src/drupalMedia/src/mediaimagetextalternative/mediaimagetextalternativeediting.js b/core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalMedia/src/mediaimagetextalternative/mediaimagetextalternativeediting.js
similarity index 100%
rename from core/assets/vendor/ckeditor5/drupal/src/drupalMedia/src/mediaimagetextalternative/mediaimagetextalternativeediting.js
rename to core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalMedia/src/mediaimagetextalternative/mediaimagetextalternativeediting.js
diff --git a/core/assets/vendor/ckeditor5/drupal/src/drupalMedia/src/mediaimagetextalternative/mediaimagetextalternativeui.js b/core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalMedia/src/mediaimagetextalternative/mediaimagetextalternativeui.js
similarity index 100%
rename from core/assets/vendor/ckeditor5/drupal/src/drupalMedia/src/mediaimagetextalternative/mediaimagetextalternativeui.js
rename to core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalMedia/src/mediaimagetextalternative/mediaimagetextalternativeui.js
diff --git a/core/assets/vendor/ckeditor5/drupal/src/drupalMedia/src/mediaimagetextalternative/ui/textalternativeformview.js b/core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalMedia/src/mediaimagetextalternative/ui/textalternativeformview.js
similarity index 99%
rename from core/assets/vendor/ckeditor5/drupal/src/drupalMedia/src/mediaimagetextalternative/ui/textalternativeformview.js
rename to core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalMedia/src/mediaimagetextalternative/ui/textalternativeformview.js
index b4062264f5..9cdd784748 100644
--- a/core/assets/vendor/ckeditor5/drupal/src/drupalMedia/src/mediaimagetextalternative/ui/textalternativeformview.js
+++ b/core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalMedia/src/mediaimagetextalternative/ui/textalternativeformview.js
@@ -1,3 +1,5 @@
+// cspell:ignore focusables
+
 import {
   ButtonView,
   FocusCycler,
diff --git a/core/assets/vendor/ckeditor5/drupal/src/drupalMedia/src/ui/utils.js b/core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalMedia/src/ui/utils.js
similarity index 100%
rename from core/assets/vendor/ckeditor5/drupal/src/drupalMedia/src/ui/utils.js
rename to core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalMedia/src/ui/utils.js
diff --git a/core/assets/vendor/ckeditor5/drupal/src/drupalMedia/src/utils.js b/core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalMedia/src/utils.js
similarity index 100%
rename from core/assets/vendor/ckeditor5/drupal/src/drupalMedia/src/utils.js
rename to core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalMedia/src/utils.js
diff --git a/core/assets/vendor/ckeditor5/drupal/src/drupalMedia/theme/icons/medialibrary.svg b/core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalMedia/theme/icons/medialibrary.svg
similarity index 100%
rename from core/assets/vendor/ckeditor5/drupal/src/drupalMedia/theme/icons/medialibrary.svg
rename to core/assets/vendor/ckeditor5/ckeditor5_plugins/drupalMedia/theme/icons/medialibrary.svg
diff --git a/core/assets/vendor/ckeditor5/drupal/README.md b/core/assets/vendor/ckeditor5/drupal/README.md
deleted file mode 100644
index 2f42f6d7b1..0000000000
--- a/core/assets/vendor/ckeditor5/drupal/README.md
+++ /dev/null
@@ -1,24 +0,0 @@
-CKEditor 5 build tools
-----------------------
-
-This module contains .js files necessary for CKEditor 5.
-These files are all contained in js/build.
-
-The original files are created by downloading the ckeditor5 repo
-running `yarn install` and `yarn dll:build`.
-
-Inside the ckeditor5 repo, the files are in individual package folders,
-for example: `packages/ckeditor5-basic-styles/build/basic-styles.js`
-
-
-Drupal specific CKEditor 5 plugins
-----------------------------------
-
-The `yarn dll:build` command generates a json manifest.
-In the ckeditor5 repo it is in `build/ckeditor5-dll.manifest.json`
-When building a custom CKEditor 5 plugin, it must reference this file in order
-to correctly set any dependency paths.
-
-This module currently has custom plugins for images and drupal media.
-The source js is in `js/drupal/src/drupalImage` or `js/drupal/src/drupalMedia`
-The built js loaded by Drupal is in `js/build/drupal`
diff --git a/core/assets/vendor/ckeditor5/scripts/build.js b/core/assets/vendor/ckeditor5/scripts/build.js
index e869cf91a9..e3845f37b4 100644
--- a/core/assets/vendor/ckeditor5/scripts/build.js
+++ b/core/assets/vendor/ckeditor5/scripts/build.js
@@ -101,6 +101,10 @@ const getTmpDir = () => {
         pack: 'ckeditor5-list',
         files: ['build/list.js'],
       },
+      {
+        pack: 'ckeditor5-paste-from-office',
+        files: ['build/paste-from-office.js'],
+      },
       {
         pack: 'ckeditor5-remove-format',
         files: ['build/remove-format.js'],
diff --git a/core/assets/vendor/ckeditor5/drupal/scripts/manifest.js b/core/assets/vendor/ckeditor5/scripts/manifest.js
similarity index 100%
rename from core/assets/vendor/ckeditor5/drupal/scripts/manifest.js
rename to core/assets/vendor/ckeditor5/scripts/manifest.js
diff --git a/core/core.libraries.yml b/core/core.libraries.yml
index 44e97c1bc3..6bf4f34cd0 100644
--- a/core/core.libraries.yml
+++ b/core/core.libraries.yml
@@ -151,6 +151,15 @@ ckeditor5.removeFormat:
   dependencies:
     - core/ckeditor5
 
+ckeditor5.pasteFromOffice:
+  remote: *ckeditor5Remote
+  version: *ckeditor5Version
+  license: *ckeditor5License
+  js:
+    assets/vendor/ckeditor5/build/paste-from-office.js: { preprocess: false, minified: true }
+  dependencies:
+    - ckeditor5/ckeditor5
+
 ckeditor5.indent:
   remote: *ckeditor5Remote
   version: *ckeditor5Version
diff --git a/core/modules/ckeditor5/ckeditor5.api.php b/core/modules/ckeditor5/ckeditor5.api.php
new file mode 100644
index 0000000000..583c164772
--- /dev/null
+++ b/core/modules/ckeditor5/ckeditor5.api.php
@@ -0,0 +1,214 @@
+<?php
+
+/**
+ * @file
+ * Documentation related to CKEditor 5.
+ */
+
+use Drupal\ckeditor5\Plugin\CKEditor5PluginDefinition;
+
+/**
+ * @defgroup ckeditor5_architecture CKEditor 5 architecture
+ * @{
+ *
+ * @section overview Overview
+ * The CKEditor 5 module integrates CKEditor 5 with Drupal's filtering and text
+ * editor APIs.
+ *
+ * Where possible, it uses upstream CKEditor plugins, but it also relies on
+ * Drupal-specific CKEditor plugins to ensure a consistent user experience.
+ *
+ * @see https://ckeditor.com/ckeditor-5/
+ *
+ * @section data_models Data models
+ * Drupal and CKEditor 5 have very different data models.
+ *
+ * Drupal stores blobs of HTML that remains manageable thanks to the use of
+ * filters and granular HTML restrictions — crucially this remains manageable
+ * thanks to those restrictions but also because Drupal does not need to
+ * process, render, understand or otherwise interact with it.
+ *
+ * @see \Drupal\text\Plugin\Field\FieldType\TextItemBase
+ * @see \Drupal\filter\Plugin\Filter\FilterInterface::getHTMLRestrictions()
+ *
+ * On the other hand, CKEditor 5 must not only be able to render these
+ * blobs, but also allow editing and creating it. This requires a much deeper
+ * understanding of that HTML.
+ *
+ * CKEditor 5 (in contrast with CKEditor 4) therefore has its own data model to
+ * represent this information — that data model is explicitly not HTML.
+ *
+ * Therefore all interactions between Drupal and CKEditor 5 need to translate
+ * between these different data models.
+ *
+ * @see https://ckeditor.com/docs/ckeditor5/latest/framework/guides/architecture/editing-engine.html#element-types-and-custom-data
+ *
+ * @section plugins CKEditor 5 Plugins
+ * CKEditor 5 plugins may use either YAML or a PHP annotation for their
+ * definitions. A PHP class does not need an annotation if it is defined in yml.
+ *
+ * To be discovered, YAML definition files must be named
+ * {module_name}.ckeditor5.yml.
+ *
+ * @see ckeditor5.ckeditor5.yml for many examples of CKEditor 5 plugin
+ * configuration as YAML.
+ *
+ * The minimally required metadata: the CKEditor 5 plugins to load, the label
+ * and the HTML elements it can generate — here's an example for a module
+ * providing a Marquee plugin, both in yml and Annotation form:
+ *
+ * @code
+ * MODULE_NAME_marquee:
+ *   ckeditor5:
+ *     plugins: [PACKAGE.CLASS]
+ *   drupal:
+ *     label: Marquee
+ *     elements:
+ *     - <marquee>
+ * @endcode
+ *
+ * and
+ *
+ * @code
+ *  * @CKEditor5Plugin(
+ *  *   id = "MODULE_NAME_marquee",
+ *  *   ckeditor5 = @CKEditor5AspectsOfCKEditor5Plugin(
+ *  *     plugins = { "PACKAGE.CLASS" },
+ *  *   ),
+ *  *   drupal = @DrupalAspectsOfCKEditor5Plugin(
+ *  *     label = @Translation("Marquee"),
+ *  *     elements = { "<marquee>" },
+ *  *   )
+ *  * )
+ * @endcode
+ *
+ * The metadata relating strictly to the CKEditor 5 plugin's JS code is stored
+ * in the 'ckeditor5' key; all other metadata is stored in the 'drupal' key.
+ *
+ * If the plugin has a dependency on another module, adding the 'provider' key
+ * will prevent the plugin from being loaded if that module is not installed.
+ *
+ * All of these can be defined in YAML or annotations. A given plugin should
+ * choose one or the other, as a definition can't parse both at once.
+ *
+ * Overview of all available plugin definition properties:
+ *
+ * - provider: Allows a plugin to have a dependency on another module. If it has
+ *   a value, a module with a machine name matching that value must be installed
+ *   for the configured plugin to load.
+ * - ckeditor5.plugins: A list CKEditor 5 JavaScript plugins to load, as
+ *   '{package.Class}' , such as 'drupalMedia.DrupalMedia'.
+ * - ckeditor5.config: A keyed array of additional values for the constructor of
+ *   the CKEditor 5 JavaScript plugins being loaded. i.e. this becomes the
+ *   CKEditor 5 plugin configuration settings (see
+ *   https://ckeditor.com/docs/ckeditor5/latest/builds/guides/integration/configuration.html)
+ *   for a given plugin.
+ * - drupal.label: Human-readable name of the CKEditor 5 plugin.
+ * - drupal.library: A Drupal asset library to load with the plugin.
+ * - drupal.admin_library: A Drupal asset library that will load in the text
+ *   format admin UI when the plugin is available.
+ * - drupal.class: Optional PHP class that makes it possible for the plugin to
+ *   provide dynamic values, or a configuration UI. The value should be
+ *   formatted as '\Drupal\{module_name}\Plugin\CKEditor5Plugin\{class_name}' to
+ *   make it discoverable.
+ * - drupal.elements: A list of elements and attributes the plugin allows use of
+ *   within CKEditor 5. This uses the same syntax as the 'filter_html' plugin
+ *   with an additional special keyword: '<$block>' . Using
+ *   '<$block [attribute(s)]>`  will permit the provided attributes in all block
+ *   level tags that are explicitly enabled in any plugin. i.e. if only '<p>',
+ *   '<h3>' and '<h2>' tags are allowed, then '<$block data-something>'  will
+ *   allow the 'data-something' attribute for '<p>', '<h3>' and '<h2>' tags.
+ * - drupal.toolbar_items: List of toolbar items the plugin provides. Keyed by a
+ *   machine name and the value being a pair defining the label:
+ *   @code
+ *   toolbar_items:
+ *     indent:
+ *       label: Indent
+ *     outdent:
+ *       label: Outdent
+ *   @encode
+ * - drupal.conditions: Conditions required for the plugin to load (other than
+ *   module dependencies, which are defined by the 'provider' property).
+ *   Conditions can check for three different things:
+ *   - 'toolbarItem': a toolbar item that must be enabled
+ *   - 'filter': a filter that must be enabled
+ *   - 'imageUploadStatus': TRUE if image upload must be enabled, FALSE if it
+ *      must not be enabled
+ *
+ * All of these can be defined in YAML or annotations. A given plugin should
+ * choose one or the other, as a definition can't parse both at once.
+ *
+ * @see \Drupal\ckeditor5\Annotation\CKEditor5Plugin
+ * @see \Drupal\ckeditor5\Annotation\CKEditor5AspectsOfCKEditor5Plugin
+ * @see \Drupal\ckeditor5\Annotation\DrupalAspectsOfCKEditor5Plugin
+ *
+ * @section upgrade_path Upgrade path
+ *
+ * Modules can provide upgrade paths similar to the built-in upgrade path for
+ * Drupal core's CKEditor 4 to CKEditor 5, by providing a CKEditor4To5Upgrade
+ * plugin. This plugin type allows:
+ * - mapping a CKEditor 4 button to an equivalent CKEditor 5 toolbar item
+ * - mapping CKEditor 4 plugin settings to equivalent CKEditor 5 plugin
+ *   configuration.
+ * The supported CKEditor 4 buttons and/or CKEditor 4 plugin settings must be
+ * specified in the annotation.
+ * See Drupal core's implementation for an example.
+ *
+ * @see \Drupal\ckeditor5\Annotation\CKEditor4To5Upgrade
+ * @see \Drupal\ckeditor5\Plugin\CKEditor4To5UpgradePluginInterface
+ * @see \Drupal\ckeditor5\Plugin\CKEditor4To5Upgrade\Core
+ *
+ * @}
+ */
+
+/**
+ * @addtogroup hooks
+ * @{
+ */
+
+/**
+ * Modify the list of available CKEditor 5 plugins.
+ *
+ * This hook may be used to modify plugin properties after they have been
+ * specified by other modules.
+ *
+ * @param array $plugin_definitions
+ *   An array of all the existing plugin definitions, passed by reference.
+ *
+ * @see \Drupal\ckeditor5\Plugin\CKEditor5PluginManager
+ */
+function hook_ckeditor5_plugin_info_alter(array &$plugin_definitions): void {
+  assert($plugin_definitions['ckeditor5_link'] instanceof CKEditor5PluginDefinition);
+  $link_plugin_definition = $plugin_definitions['ckeditor5_link']->toArray();
+  $link_plugin_definition['drupal']['config']['link']['decorators'][] = [
+    'mode' => 'manual',
+    'label' => t('Open in new window'),
+    'attributes' => [
+      'target' => '_blank',
+    ],
+  ];
+  $plugin_definitions['ckeditor5_link'] = new CKEditor5PluginDefinition($link_plugin_definition);
+
+}
+
+/**
+ * Modify the list of available CKEditor 4 to 5 Upgrade plugins.
+ *
+ * This hook may be used to modify plugin properties after they have been
+ * specified by other modules. For example, to override a default upgrade path.
+ *
+ * @param array $plugin_definitions
+ *   An array of all the existing plugin definitions, passed by reference.
+ *
+ * @see \Drupal\ckeditor5\Plugin\CKEditor4To5UpgradePluginManager
+ */
+function hook_ckeditor4to5upgrade_plugin_info_alter(array &$plugin_definitions): void {
+  // Remove core's upgrade path for the "Maximize" button (which is: there is no
+  // equivalent). This allows a different CKEditor4To5Upgrade plugin to define
+  // this upgrade path instead.
+  unset($plugin_definitions['core']['cke4_buttons']['Maximize']);
+}
+
+/**
+ * @} End of "addtogroup hooks".
+ */
diff --git a/core/modules/ckeditor5/ckeditor5.ckeditor5.yml b/core/modules/ckeditor5/ckeditor5.ckeditor5.yml
index 27e79401b3..81761927c8 100644
--- a/core/modules/ckeditor5/ckeditor5.ckeditor5.yml
+++ b/core/modules/ckeditor5/ckeditor5.ckeditor5.yml
@@ -307,6 +307,15 @@ ckeditor5_removeFormat:
         label: Remove Format
     elements: false
 
+ckeditor5_pasteFromOffice:
+  ckeditor5:
+    plugins: [pasteFromOffice.PasteFromOffice]
+  drupal:
+    label: Paste From Office
+    library: core/ckeditor5.pasteFromOffice
+    elements: false
+    conditions: []
+
 ckeditor5_table:
   ckeditor5:
     plugins:
diff --git a/core/modules/ckeditor5/ckeditor5.module b/core/modules/ckeditor5/ckeditor5.module
index d5e2f8a994..a81cc85bce 100644
--- a/core/modules/ckeditor5/ckeditor5.module
+++ b/core/modules/ckeditor5/ckeditor5.module
@@ -185,7 +185,7 @@ function ckeditor5_form_filter_format_form_alter(array &$form, FormStateInterfac
      * @param array $plugins_config_form
      *   The plugins config subform render array.
      */
-    function _add_ajax_listeners_to_plugin_inputs(array &$plugins_config_form) : void {
+    function _add_ajax_listeners_to_plugin_inputs(array &$plugins_config_form): void {
       $field_types = [
         'checkbox',
         'select',
@@ -279,7 +279,7 @@ function _update_ckeditor5_html_filter(array $form, FormStateInterface $form_sta
      * @param \Drupal\Core\Ajax\AjaxResponse $response
      *   The AJAX response attachments will be added to.
      */
-    function _add_attachments_to_editor_update_response(array $form, AjaxResponse &$response) : void {
+    function _add_attachments_to_editor_update_response(array $form, AjaxResponse &$response): void {
       foreach ($form as $key => $value) {
         if ($key === "#attached") {
           $response->addAttachments(array_diff_key($value, ['placeholders' => '']));
diff --git a/core/modules/ckeditor5/ckeditor5.services.yml b/core/modules/ckeditor5/ckeditor5.services.yml
index ca4911d3aa..f9f4f17149 100644
--- a/core/modules/ckeditor5/ckeditor5.services.yml
+++ b/core/modules/ckeditor5/ckeditor5.services.yml
@@ -2,7 +2,13 @@ services:
   plugin.manager.ckeditor5.plugin:
     class: Drupal\ckeditor5\Plugin\CKEditor5PluginManager
     parent: default_plugin_manager
+  # @todo Remove in Drupal 11: https://www.drupal.org/project/ckeditor5/issues/3239012
+  plugin.manager.ckeditor4to5upgrade.plugin:
+    class: Drupal\ckeditor5\Plugin\CKEditor4To5UpgradePluginManager
+    parent: default_plugin_manager
   ckeditor5.smart_default_settings:
     class: Drupal\ckeditor5\SmartDefaultSettings
     arguments:
       - '@plugin.manager.ckeditor5.plugin'
+      - '@plugin.manager.ckeditor4to5upgrade.plugin'
+      - '@?plugin.manager.ckeditor.plugin'
diff --git a/core/modules/ckeditor5/src/Annotation/CKEditor4To5Upgrade.php b/core/modules/ckeditor5/src/Annotation/CKEditor4To5Upgrade.php
new file mode 100644
index 0000000000..f8fd02b673
--- /dev/null
+++ b/core/modules/ckeditor5/src/Annotation/CKEditor4To5Upgrade.php
@@ -0,0 +1,43 @@
+<?php
+
+declare(strict_types = 1);
+
+namespace Drupal\ckeditor5\Annotation;
+
+use Drupal\Component\Annotation\Plugin;
+
+/**
+ * Defines a CKEditor4To5Upgrade annotation object.
+ *
+ * Plugin Namespace: Plugin\CKEditor4To5Upgrade.
+ *
+ * @see \Drupal\ckeditor5\Plugin\CKEditor5PluginInterface
+ * @see \Drupal\ckeditor5\Plugin\CKEditor5PluginManagerInterface
+ * @see plugin_api
+ *
+ * @Annotation
+ */
+class CKEditor4To5Upgrade extends Plugin {
+
+  /**
+   * The plugin ID.
+   *
+   * @var string
+   */
+  public $id;
+
+  /**
+   * The CKEditor 4 buttons for which an upgrade path is provided.
+   *
+   * @var string[]
+   */
+  public $cke4_buttons;
+
+  /**
+   * The CKEditor 4 plugins for whose settings an upgrade path is provided.
+   *
+   * @var string[]
+   */
+  public $cke4_plugin_settings;
+
+}
diff --git a/core/modules/ckeditor5/src/Controller/CKEditor5MediaController.php b/core/modules/ckeditor5/src/Controller/CKEditor5MediaController.php
index d05b9f3b92..fa51000453 100644
--- a/core/modules/ckeditor5/src/Controller/CKEditor5MediaController.php
+++ b/core/modules/ckeditor5/src/Controller/CKEditor5MediaController.php
@@ -119,7 +119,7 @@ public function isMediaImage(Request $request) {
    * @return \Drupal\Core\Access\AccessResultInterface
    *   The access result.
    */
-  public function access(Editor $editor) : AccessResultInterface {
+  public function access(Editor $editor): AccessResultInterface {
     if ($editor->getEditor() !== 'ckeditor5') {
       return AccessResult::forbidden();
     }
diff --git a/core/modules/ckeditor5/src/HTMLRestrictionsUtilities.php b/core/modules/ckeditor5/src/HTMLRestrictionsUtilities.php
index d0d43a80bf..dee7d24c50 100644
--- a/core/modules/ckeditor5/src/HTMLRestrictionsUtilities.php
+++ b/core/modules/ckeditor5/src/HTMLRestrictionsUtilities.php
@@ -34,7 +34,7 @@ final class HTMLRestrictionsUtilities {
    * @return string
    *   A formatted list; a string representation of the given HTML elements.
    */
-  public static function toReadableElements(array $elements) : array {
+  public static function toReadableElements(array $elements): array {
     $readable = [];
     foreach ($elements as $tag => $attributes) {
       $attribute_string = '';
@@ -65,7 +65,7 @@ public static function toReadableElements(array $elements) : array {
    * @return string[]
    *   A list of strings, with a HTML tag and potentially attributes in each.
    */
-  public static function allowedElementsStringToPluginElementsArray(string $elements_string) : array {
+  public static function allowedElementsStringToPluginElementsArray(string $elements_string): array {
     $html_restrictions = static::allowedElementsStringToHtmlFilterArray($elements_string);
     return static::toReadableElements($html_restrictions);
   }
@@ -82,7 +82,7 @@ public static function allowedElementsStringToPluginElementsArray(string $elemen
    * @see \Drupal\ckeditor5\Plugin\CKEditor5PluginManager::WILDCARD_ELEMENT_METHODS
    *   Each key in this array represents a valid wildcard tag.
    */
-  public static function allowedElementsStringToHtmlFilterArray(string $elements_string) : array {
+  public static function allowedElementsStringToHtmlFilterArray(string $elements_string): array {
     preg_match('/<(\$[A-Z,a-z]*)/', $elements_string, $wildcard_matches);
 
     $wildcard = NULL;
@@ -126,7 +126,7 @@ public static function allowedElementsStringToHtmlFilterArray(string $elements_s
    * @return array
    *   The array without unwanted artifacts.
    */
-  public static function cleanAllowedHtmlArray(array $elements) : array {
+  public static function cleanAllowedHtmlArray(array $elements): array {
     // When recursively merging elements arrays, unkeyed boolean values can
     // appear in attribute config arrays. This removes them.
     foreach ($elements as $tag => $tag_config) {
@@ -149,7 +149,7 @@ public static function cleanAllowedHtmlArray(array $elements) : array {
    * @param array|bool $value
    *   The attribute config value.
    */
-  public static function providedElementsAttributes(array &$elements, string $tag, string $attribute, $value) : void {
+  public static function providedElementsAttributes(array &$elements, string $tag, string $attribute, $value): void {
     $attribute_already_allows_all = isset($elements[$tag][$attribute]) && $elements[$tag][$attribute] === TRUE;
 
     if ($value === TRUE) {
@@ -174,7 +174,7 @@ public static function providedElementsAttributes(array &$elements, string $tag,
    *   Returns an array with all the values in $elements_array_1 that are not
    *   present in $elements_array_1, including values that are FALSE
    */
-  public static function diffAllowedElements(array $elements_array_1, array $elements_array_2) : array {
+  public static function diffAllowedElements(array $elements_array_1, array $elements_array_2): array {
     return array_filter(
       DiffArray::diffAssocRecursive($elements_array_1, $elements_array_2),
       // DiffArray::diffAssocRecursive() does not know the semantics of the
@@ -200,7 +200,7 @@ function ($value, string $tag) use ($elements_array_2) {
    *
    * @see https://ckeditor5.github.io/docs/nightly/ckeditor5/latest/features/general-html-support.html#configuration
    */
-  public static function allowedElementsStringToHtmlSupportConfig(string $elements_string) : array {
+  public static function allowedElementsStringToHtmlSupportConfig(string $elements_string): array {
     $html_restrictions = static::allowedElementsStringToHtmlFilterArray($elements_string);
     $allowed = [];
     foreach ($html_restrictions as $tag => $attributes) {
diff --git a/core/modules/ckeditor5/src/Plugin/CKEditor4To5Upgrade/Core.php b/core/modules/ckeditor5/src/Plugin/CKEditor4To5Upgrade/Core.php
new file mode 100644
index 0000000000..c6956df35f
--- /dev/null
+++ b/core/modules/ckeditor5/src/Plugin/CKEditor4To5Upgrade/Core.php
@@ -0,0 +1,179 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\ckeditor5\Plugin\CKEditor4To5Upgrade;
+
+use Drupal\ckeditor5\Plugin\CKEditor4To5UpgradePluginInterface;
+use Drupal\Core\Plugin\PluginBase;
+
+/**
+ * Provides the CKEditor 4 to 5 upgrade for Drupal core's CKEditor plugins.
+ *
+ * @CKEditor4To5Upgrade(
+ *   id = "core",
+ *   cke4_buttons = {
+ *     "DrupalImage",
+ *     "DrupalLink",
+ *     "DrupalUnlink",
+ *     "Bold",
+ *     "Italic",
+ *     "Underline",
+ *     "Superscript",
+ *     "Subscript",
+ *     "BulletedList",
+ *     "NumberedList",
+ *     "Outdent",
+ *     "Indent",
+ *     "Undo",
+ *     "Redo",
+ *     "Blockquote",
+ *     "JustifyLeft",
+ *     "JustifyCenter",
+ *     "JustifyRight",
+ *     "JustifyBlock",
+ *     "HorizontalRule",
+ *     "Format",
+ *     "Table",
+ *     "Source",
+ *     "Strike",
+ *     "Cut",
+ *     "Copy",
+ *     "Paste",
+ *     "PasteText",
+ *     "PasteFromWord",
+ *     "ShowBlocks",
+ *     "Maximize",
+ *     "-",
+ *     "RemoveFormat",
+ *     "Styles",
+ *     "SpecialChar",
+ *     "Language",
+ *     "DrupalMediaLibrary",
+ *   },
+ *   cke4_plugin_settings = {
+ *     "stylescombo",
+ *     "language",
+ *   }
+ * )
+ */
+class Core extends PluginBase implements CKEditor4To5UpgradePluginInterface {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function mapCKEditor4ToolbarButtonToCKEditor5ToolbarItem(string $cke4_button): ?string {
+    switch ($cke4_button) {
+      // @see \Drupal\ckeditor\Plugin\CKEditorPlugin\DrupalImage
+      case 'DrupalImage':
+        return 'uploadImage';
+
+      // @see \Drupal\ckeditor\Plugin\CKEditorPlugin\DrupalLink
+      case 'DrupalLink':
+        return 'link';
+
+      case 'DrupalUnlink':
+        return NULL;
+
+      // @see \Drupal\ckeditor\Plugin\CKEditorPlugin\Internal
+      case 'Bold':
+      case 'Italic':
+      case 'Underline':
+      case 'Superscript':
+      case 'Subscript':
+      case 'BulletedList':
+      case 'NumberedList':
+      case 'Outdent':
+      case 'Indent':
+      case 'Undo':
+      case 'Redo':
+        return lcfirst($cke4_button);
+
+      case 'Blockquote':
+        return 'blockQuote';
+
+      case 'JustifyLeft':
+        return "alignment:left";
+
+      case 'JustifyCenter':
+        return "alignment:center";
+
+      case 'JustifyRight':
+        return "alignment:right";
+
+      case 'JustifyBlock':
+        return "alignment:justify";
+
+      case 'HorizontalRule':
+        return 'horizontalLine';
+
+      case 'Format':
+        return 'heading';
+
+      case 'Table':
+        return 'insertTable';
+
+      case 'Source':
+        return 'sourceEditing';
+
+      case 'Strike':
+        return 'strikethrough';
+
+      case 'Cut':
+      case 'Copy':
+      case 'Paste':
+      case 'PasteText':
+      case 'PasteFromWord':
+      case 'ShowBlocks':
+      case 'Maximize':
+      case '-':
+        // @see https://www.drupal.org/project/ckeditor5/issues/3211049#comment-14167764
+        return NULL;
+
+      // @see \Drupal\ckeditor5\Plugin\CKEditor5Plugin\RemoveFormat
+      case 'RemoveFormat':
+        return 'removeFormat';
+
+      // @see \Drupal\ckeditor\Plugin\CKEditorPlugin\StylesCombo
+      case 'Styles':
+        // @todo Change in https://www.drupal.org/project/ckeditor5/issues/3222797
+        return NULL;
+
+      // @see \Drupal\ckeditor5\Plugin\CKEditor5Plugin\specialCharacters
+      case 'SpecialChar':
+        return 'specialCharacters';
+
+      // @see \Drupal\ckeditor\Plugin\CKEditorPlugin\Language
+      case 'Language':
+        return 'textPartLanguage';
+
+      // @see \Drupal\media_library\Plugin\CKEditorPlugin\DrupalMediaLibrary
+      case 'DrupalMediaLibrary':
+        return 'drupalMedia';
+
+      default:
+        throw new \OutOfBoundsException();
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function mapCKEditor4SettingsToCKEditor5Configuration(string $cke4_plugin_id, array $cke4_plugin_settings): ?array {
+    switch ($cke4_plugin_id) {
+      // @see \Drupal\ckeditor\Plugin\CKEditorPlugin\StylesCombo
+      case 'stylescombo':
+        // @todo Change in https://www.drupal.org/project/ckeditor5/issues/3222797
+        return NULL;
+
+      // @see \Drupal\ckeditor\Plugin\CKEditorPlugin\Language
+      case 'language':
+        // Identical configuration.
+        return ['ckeditor5_language' => $cke4_plugin_settings];
+
+      default:
+        throw new \OutOfBoundsException();
+    }
+  }
+
+}
diff --git a/core/modules/ckeditor5/src/Plugin/CKEditor4To5UpgradePluginInterface.php b/core/modules/ckeditor5/src/Plugin/CKEditor4To5UpgradePluginInterface.php
new file mode 100644
index 0000000000..82541216d2
--- /dev/null
+++ b/core/modules/ckeditor5/src/Plugin/CKEditor4To5UpgradePluginInterface.php
@@ -0,0 +1,66 @@
+<?php
+
+declare(strict_types = 1);
+
+namespace Drupal\ckeditor5\Plugin;
+
+use Drupal\Component\Plugin\PluginInspectionInterface;
+
+/**
+ * Defines an interface for CKEditor 4 to 5 upgrade plugins.
+ *
+ * @see \Drupal\ckeditor5\Plugin\CKEditor4To5UpgradePluginManager
+ * @see \Drupal\ckeditor5\Annotation\CKEditor4To5Upgrade
+ * @see plugin_api
+ */
+interface CKEditor4To5UpgradePluginInterface extends PluginInspectionInterface {
+
+  /**
+   * Maps a CKEditor 4 button to the CKEditor 5 equivalent, if it exists.
+   *
+   * Generated by inspecting all \Drupal\ckeditor\CKEditorPluginButtonsInterface
+   * implementations.
+   *
+   * @param string $cke4_button
+   *   A valid CKEditor 4 button name.
+   *
+   * @return string|null
+   *   The equivalent CKEditor 5 toolbar item, or NULL if no equivalent exists.
+   *   In either case, the button name must be added to the annotation.
+   *
+   * @throws \OutOfBoundsException
+   *   Thrown when this plugin does not know whether an equivalent exists.
+   *
+   * @see \Drupal\ckeditor\CKEditorPluginButtonsInterface
+   * @see \Drupal\ckeditor5\Annotation\CKEditor4To5Upgrade
+   */
+  public function mapCKEditor4ToolbarButtonToCKEditor5ToolbarItem(string $cke4_button): ?string;
+
+  /**
+   * Maps CKEditor 4 settings to the CKEditor 5 equivalent, if needed.
+   *
+   * Not every CKEditor 5 plugin has settings; some CKEditor 5 plugins may have
+   * settings that the CKEditor 4 equivalent did not and vice versa. Therefore
+   * the complete CKEditor 4 settings are provided, and any CKEditor 5 setting
+   * can be set.
+   *
+   * @param string $cke4_plugin_id
+   *   The CKEditor 4 plugin whose settings need to be mapped.
+   * @param array $cke4_plugin_settings
+   *   The settings for this CKEditor 4 plugin.
+   *
+   * @return array|null
+   *   NULL if not needed, otherwise an array with a single key-value pair:
+   *   - key: the plugin ID of the equivalent CKEditor 5 plugin
+   *   - value: the equivalent settings
+   *   In either case, the button name must be added to the annotation.
+   *
+   * @throws \OutOfBoundsException
+   *   Thrown when this plugin does not know whether an equivalent exists.
+   *
+   * @see \Drupal\ckeditor\CKEditorPluginConfigurableInterface
+   * @see \Drupal\ckeditor5\Annotation\CKEditor4To5Upgrade
+   */
+  public function mapCKEditor4SettingsToCKEditor5Configuration(string $cke4_plugin_id, array $cke4_plugin_settings): ?array;
+
+}
diff --git a/core/modules/ckeditor5/src/Plugin/CKEditor4To5UpgradePluginManager.php b/core/modules/ckeditor5/src/Plugin/CKEditor4To5UpgradePluginManager.php
new file mode 100644
index 0000000000..55375574bb
--- /dev/null
+++ b/core/modules/ckeditor5/src/Plugin/CKEditor4To5UpgradePluginManager.php
@@ -0,0 +1,165 @@
+<?php
+
+declare(strict_types = 1);
+
+namespace Drupal\ckeditor5\Plugin;
+
+use Drupal\ckeditor5\Annotation\CKEditor4To5Upgrade;
+use Drupal\Component\Assertion\Inspector;
+use Drupal\Core\Cache\CacheBackendInterface;
+use Drupal\Core\Extension\ModuleHandlerInterface;
+use Drupal\Core\Plugin\DefaultPluginManager;
+
+/**
+ * Provides a CKEditor 4 to 5 upgrade plugin manager.
+ *
+ * @see \Drupal\ckeditor5\Plugin\CKEditor4To5UpgradePluginInterface
+ * @see \Drupal\ckeditor5\Annotation\CKEditor4To5Upgrade
+ * @see plugin_api
+ */
+class CKEditor4To5UpgradePluginManager extends DefaultPluginManager {
+
+  /**
+   * A map of CKEditor 4 buttons to an upgrade plugin ID.
+   *
+   * @var array
+   */
+  protected $cke4ButtonsMap;
+
+  /**
+   * A map of CKEditor 4 plugins with settings to an upgrade plugin ID.
+   *
+   * @var array
+   */
+  protected $cke4PluginSettingsMap;
+
+  /**
+   * Constructs a CKEditor4To5UpgradePluginManager object.
+   *
+   * @param \Traversable $namespaces
+   *   An object that implements \Traversable which contains the root paths
+   *   keyed by the corresponding namespace to look for plugin implementations.
+   * @param \Drupal\Core\Cache\CacheBackendInterface $cache_backend
+   *   Cache backend instance to use.
+   * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
+   *   The module handler to invoke the alter hook with.
+   */
+  public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, ModuleHandlerInterface $module_handler) {
+    parent::__construct('Plugin/CKEditor4To5Upgrade', $namespaces, $module_handler, CKEditor4To5UpgradePluginInterface::class, CKEditor4To5Upgrade::class);
+
+    $this->alterInfo('ckeditor4to5upgrade_plugin_info');
+    $this->setCacheBackend($cache_backend, 'ckeditor4to5upgrade_plugins');
+  }
+
+  /**
+   * Validates plugin definitions: avoids conflicts. Builds maps for later use.
+   */
+  protected function validateAndBuildMaps(): void {
+    if ($this->cke4ButtonsMap !== NULL) {
+      return;
+    }
+
+    foreach ($this->getDefinitions() as $upgrade_plugin_id => $definition) {
+      // Only one CKEditor4To5Upgrade plugin can define the upgrade path for a
+      // CKEditor 4 button.
+      if (isset($definition['cke4_buttons'])) {
+        assert(Inspector::assertAllStrings($definition['cke4_buttons']));
+        foreach ($definition['cke4_buttons'] as $cke4_button_id) {
+          if (isset($this->cke4ButtonsMap[$cke4_button_id])) {
+            throw new \OutOfBoundsException(sprintf('The "%s" CKEditor 4 button is already being upgraded by the "%s" CKEditor4To5Upgrade plugin, the "%s" plugin is as well. This conflict needs to be resolved.', $cke4_button_id, $this->cke4ButtonsMap[$cke4_button_id], $upgrade_plugin_id));
+          }
+          $this->cke4ButtonsMap[$cke4_button_id] = $upgrade_plugin_id;
+        }
+      }
+
+      // Only one CKEditor4To5Upgrade plugin can define the upgrade path for a
+      // CKEditor 4 plugin's settings.
+      if (isset($definition['cke4_plugin_settings'])) {
+        assert(Inspector::assertAllStrings($definition['cke4_plugin_settings']));
+        foreach ($definition['cke4_plugin_settings'] as $cke4_plugin_id) {
+          if (isset($this->cke4PluginSettingsMap[$cke4_plugin_id])) {
+            throw new \OutOfBoundsException(sprintf('The "%s" CKEditor 4 plugin\'s settings are already being upgraded by the "%s" CKEditor4To5Upgrade plugin, the "%s" plugin is as well. This conflict needs to be resolved.', $cke4_plugin_id, $this->cke4PluginSettingsMap[$cke4_plugin_id], $upgrade_plugin_id));
+          }
+          $this->cke4PluginSettingsMap[$cke4_plugin_id] = $upgrade_plugin_id;
+        }
+      }
+    }
+  }
+
+  /**
+   * Maps a CKEditor 4 button to the CKEditor 5 equivalent, if it exists.
+   *
+   * Generated by inspecting all \Drupal\ckeditor\CKEditorPluginButtonsInterface
+   * implementations.
+   *
+   * @param string $cke4_button
+   *   A valid CKEditor 4 button name.
+   *
+   * @return string|null
+   *   The equivalent CKEditor 5 toolbar item, or NULL if no equivalent exists.
+   *
+   * @throws \OutOfBoundsException
+   *   Thrown when no upgrade path exists.
+   * @throws \LogicException
+   *   Thrown when a plugin claims to provide an upgrade path but does not.
+   *
+   * @see \Drupal\ckeditor\CKEditorPluginButtonsInterface
+   */
+  public function mapCKEditor4ToolbarButtonToCKEditor5ToolbarItem(string $cke4_button): ?string {
+    $this->validateAndBuildMaps();
+
+    if (!isset($this->cke4ButtonsMap[$cke4_button])) {
+      throw new \OutOfBoundsException(sprintf('No upgrade path found for the "%s" button.', $cke4_button));
+    }
+
+    $plugin_id = $this->cke4ButtonsMap[$cke4_button];
+    try {
+      return $this->createInstance($plugin_id)->mapCKEditor4ToolbarButtonToCKEditor5ToolbarItem($cke4_button);
+    }
+    catch (\OutOfBoundsException $e) {
+      throw new \LogicException(sprintf('The "%s" CKEditor4To5Upgrade plugin claims to provide an upgrade path for the "%s" CKEditor 4 button but does not.', $plugin_id, $cke4_button));
+    }
+  }
+
+  /**
+   * Maps CKEditor 4 settings to the CKEditor 5 equivalent, if needed.
+   *
+   * Not every CKEditor 5 plugin has settings; some CKEditor 5 plugins may have
+   * settings that the CKEditor 4 equivalent did not and vice versa. Therefore
+   * the complete CKEditor 4 settings are provided, and any CKEditor 5 setting
+   * can be set.
+   *
+   * @param string $cke4_plugin_id
+   *   The CKEditor 4 plugin whose settings need to be mapped.
+   * @param array $cke4_plugin_settings
+   *   The settings for this CKEditor 4 plugin.
+   *
+   * @return array|null
+   *   NULL if not needed, otherwise an array with a single key-value pair:
+   *   - key: the plugin ID of the equivalent CKEditor 5 plugin
+   *   - value: the equivalent settings
+   *
+   * @throws \OutOfBoundsException
+   *   Thrown when no upgrade path exists.
+   * @throws \LogicException
+   *   Thrown when a plugin claims to provide an upgrade path but does not.
+   *
+   * @see \Drupal\ckeditor\CKEditorPluginConfigurableInterface
+   */
+  public function mapCKEditor4SettingsToCKEditor5Configuration(string $cke4_plugin_id, array $cke4_plugin_settings): ?array {
+    $this->validateAndBuildMaps();
+
+    if (!isset($this->cke4PluginSettingsMap[$cke4_plugin_id])) {
+      throw new \OutOfBoundsException(sprintf('No upgrade path found for the "%s" plugin settings.', $cke4_plugin_id));
+    }
+
+    $plugin_id = $this->cke4PluginSettingsMap[$cke4_plugin_id];
+    try {
+      return $this->createInstance($plugin_id)->mapCKEditor4SettingsToCKEditor5Configuration($cke4_plugin_id, $cke4_plugin_settings);
+    }
+    catch (\OutOfBoundsException $e) {
+      throw new \LogicException(sprintf('The "%s" CKEditor4To5Upgrade plugin claims to provide an upgrade path for the "%s" CKEditor 4 plugin settings but does not.', $plugin_id, $cke4_plugin_id));
+    }
+  }
+
+}
diff --git a/core/modules/ckeditor5/src/Plugin/CKEditor5Plugin/Heading.php b/core/modules/ckeditor5/src/Plugin/CKEditor5Plugin/Heading.php
index 41694cbe20..b46561e915 100644
--- a/core/modules/ckeditor5/src/Plugin/CKEditor5Plugin/Heading.php
+++ b/core/modules/ckeditor5/src/Plugin/CKEditor5Plugin/Heading.php
@@ -58,13 +58,13 @@ public function defaultConfiguration() {
    * @return string[]
    *   All valid choices.
    */
-  public static function validChoices() : array {
+  public static function validChoices(): array {
     $cke5_plugin_manager = \Drupal::service('plugin.manager.ckeditor5.plugin');
     assert($cke5_plugin_manager instanceof CKEditor5PluginManagerInterface);
-    $definition = $cke5_plugin_manager->getDefinition('ckeditor5_heading');
-    assert($definition->getClass() === static::class);
+    $plugin_definition = $cke5_plugin_manager->getDefinition('ckeditor5_heading');
+    assert($plugin_definition->getClass() === static::class);
     return array_diff(
-      array_column($definition->getCKEditor5Config()['heading']['options'], 'model'),
+      array_column($plugin_definition->getCKEditor5Config()['heading']['options'], 'model'),
       static::ALWAYS_ENABLED_HEADINGS
     );
   }
@@ -142,7 +142,7 @@ public function submitConfigurationForm(array &$form, FormStateInterface $form_s
    *
    * @see https://ckeditor.com/docs/ckeditor5/latest/api/module_heading_heading-HeadingConfig.html#member-options
    */
-  private static function generateCheckboxForHeadingOption(array $heading_option) : array {
+  private static function generateCheckboxForHeadingOption(array $heading_option): array {
     // This requires the `title` and `model` properties. The `class` property is
     // optional. The `view` property is not used.
     assert(array_key_exists('title', $heading_option));
@@ -197,11 +197,11 @@ public function getElementsSubset(): array {
    * @return string[]
    *   List of tags provided by the enabled headings.
    */
-  private function enabledHeadingsToTags(array $enabled_headings) : array {
-    $definition = $this->getPluginDefinition();
-    $elements = $definition->getElements();
+  private function enabledHeadingsToTags(array $enabled_headings): array {
+    $plugin_definition = $this->getPluginDefinition();
+    $elements = $plugin_definition->getElements();
     $heading_keyed_by_model = [];
-    foreach ($definition->getCKEditor5Config()['heading']['options'] as $configured_heading) {
+    foreach ($plugin_definition->getCKEditor5Config()['heading']['options'] as $configured_heading) {
       if (isset($configured_heading['model'])) {
         $heading_keyed_by_model[$configured_heading['model']] = $configured_heading;
       }
diff --git a/core/modules/ckeditor5/src/Plugin/CKEditor5Plugin/Language.php b/core/modules/ckeditor5/src/Plugin/CKEditor5Plugin/Language.php
index b0325e97ce..31dc6a89a3 100644
--- a/core/modules/ckeditor5/src/Plugin/CKEditor5Plugin/Language.php
+++ b/core/modules/ckeditor5/src/Plugin/CKEditor5Plugin/Language.php
@@ -45,9 +45,9 @@ public function getDynamicPluginConfig(array $static_plugin_config, EditorInterf
 
     // Sort on full language name.
     ksort($language_list);
-    $config = $static_plugin_config;
-    $config['language']['textPartLanguage'] = array_values($language_list);
-    return $config;
+    $dynamic_plugin_config = $static_plugin_config;
+    $dynamic_plugin_config['language']['textPartLanguage'] = array_values($language_list);
+    return $dynamic_plugin_config;
   }
 
   /**
diff --git a/core/modules/ckeditor5/src/Plugin/CKEditor5Plugin/Media.php b/core/modules/ckeditor5/src/Plugin/CKEditor5Plugin/Media.php
index fa23477f54..93ac358a78 100644
--- a/core/modules/ckeditor5/src/Plugin/CKEditor5Plugin/Media.php
+++ b/core/modules/ckeditor5/src/Plugin/CKEditor5Plugin/Media.php
@@ -19,17 +19,17 @@ class Media extends CKEditor5PluginDefault {
    * {@inheritdoc}
    */
   public function getDynamicPluginConfig(array $static_plugin_config, EditorInterface $editor): array {
-    $config = $static_plugin_config;
-    $config['drupalMedia']['previewURL'] = Url::fromRoute('media.filter.preview')
+    $dynamic_plugin_config = $static_plugin_config;
+    $dynamic_plugin_config['drupalMedia']['previewURL'] = Url::fromRoute('media.filter.preview')
       ->setRouteParameter('filter_format', $editor->getFilterFormat()->id())
       ->toString(TRUE)
       ->getGeneratedUrl();
-    $config['drupalMedia']['isMediaUrl'] = Url::fromRoute('ckeditor5.media_image')
+    $dynamic_plugin_config['drupalMedia']['isMediaUrl'] = Url::fromRoute('ckeditor5.media_image')
       ->setRouteParameter('editor', $editor->id())
       ->toString(TRUE)
       ->getGeneratedUrl();
 
-    return $config;
+    return $dynamic_plugin_config;
   }
 
 }
diff --git a/core/modules/ckeditor5/src/Plugin/CKEditor5Plugin/MediaLibrary.php b/core/modules/ckeditor5/src/Plugin/CKEditor5Plugin/MediaLibrary.php
index cf9d1c9868..8561a7db7d 100644
--- a/core/modules/ckeditor5/src/Plugin/CKEditor5Plugin/MediaLibrary.php
+++ b/core/modules/ckeditor5/src/Plugin/CKEditor5Plugin/MediaLibrary.php
@@ -91,9 +91,9 @@ public function getDynamicPluginConfig(array $static_plugin_config, EditorInterf
       ->toString(TRUE)
       ->getGeneratedUrl();
 
-    $config = $static_plugin_config;
-    $config['drupalMedia']['libraryURL'] = $library_url;
-    return $config;
+    $dynamic_plugin_config = $static_plugin_config;
+    $dynamic_plugin_config['drupalMedia']['libraryURL'] = $library_url;
+    return $dynamic_plugin_config;
   }
 
 }
diff --git a/core/modules/ckeditor5/src/Plugin/CKEditor5PluginDefinition.php b/core/modules/ckeditor5/src/Plugin/CKEditor5PluginDefinition.php
index 1bb191e79a..20df9dee33 100644
--- a/core/modules/ckeditor5/src/Plugin/CKEditor5PluginDefinition.php
+++ b/core/modules/ckeditor5/src/Plugin/CKEditor5PluginDefinition.php
@@ -54,6 +54,20 @@ public function __construct(array $definition) {
     $this->drupal = $definition['drupal'];
   }
 
+  /**
+   * Gets an array representation of this CKEditor 5 plugin definition.
+   *
+   * @return array
+   */
+  public function toArray(): array {
+    return [
+      'id' => $this->id(),
+      'provider' => $this->provider,
+      'ckeditor5' => $this->ckeditor5,
+      'drupal' => $this->drupal,
+    ];
+  }
+
   /**
    * Validates the CKEditor 5 aspects of the CKEditor 5 plugin definition.
    *
@@ -105,7 +119,32 @@ private static function validateDrupalAspects(string $id, array $definition): vo
       throw new InvalidPluginDefinitionException($id, sprintf('The "%s" CKEditor 5 plugin definition must contain a "drupal.elements" key.', $id));
     }
     elseif ($definition['drupal']['elements'] !== FALSE && !(is_array($definition['drupal']['elements']) && !empty($definition['drupal']['elements']) && Inspector::assertAllStrings($definition['drupal']['elements']))) {
-      throw new InvalidPluginDefinitionException($id, sprintf('The "%s" CKEditor 5 plugin definition has a "drupal.elements" key that is neither a list of HTML tags/attributes nor false.', $id));
+      throw new InvalidPluginDefinitionException($id, sprintf('The "%s" CKEditor 5 plugin definition has a "drupal.elements" value that is neither a list of HTML tags/attributes nor false.', $id));
+    }
+    elseif (is_array($definition['drupal']['elements'])) {
+      foreach ($definition['drupal']['elements'] as $index => $element) {
+        // ckeditor5_sourceEditing is the edge case here: it is the only plugin
+        // that is allowed to return a superset. It's a special case because it
+        // is through configuring this particular plugin that additional HTML
+        // tags can be allowed.
+        // Even though its plugin definition says '<*>' is supported, this is a
+        // little lie to convey that this plugin is capable of supporting any
+        // HTML tag … but which ones are actually supported depends on the
+        // configuration.
+        // This also means that without any configuration, it does not support
+        // any HTML tags.
+        // @see \Drupal\ckeditor5\Plugin\CKEditor5PluginManager::getProvidedElements()
+        if ($definition['id'] === 'ckeditor5_sourceEditing') {
+          continue;
+        }
+        $parsed_elements = HTMLRestrictionsUtilities::allowedElementsStringToPluginElementsArray($element);
+        if (count($parsed_elements) === 0) {
+          throw new InvalidPluginDefinitionException($id, sprintf('The "%s" CKEditor 5 plugin definition has a value at "drupal.elements.%d" that is not an HTML tag with optional attributes: "%s". Expected structure: "<tag allowedAttribute="allowedValue1 allowedValue2">".', $id, $index, $element));
+        }
+        elseif (count($parsed_elements) > 1) {
+          throw new InvalidPluginDefinitionException($id, sprintf('The "%s" CKEditor 5 plugin definition has a value at "drupal.elements.%d": multiple tags listed, should be one: "%s".', $id, $index, $element));
+        }
+      }
     }
 
     if (isset($definition['drupal']['class']) && !in_array(CKEditor5PluginInterface::class, class_implements($definition['drupal']['class']))) {
diff --git a/core/modules/ckeditor5/src/Plugin/CKEditor5PluginManager.php b/core/modules/ckeditor5/src/Plugin/CKEditor5PluginManager.php
index 1ea3345ec2..13a28aa19a 100644
--- a/core/modules/ckeditor5/src/Plugin/CKEditor5PluginManager.php
+++ b/core/modules/ckeditor5/src/Plugin/CKEditor5PluginManager.php
@@ -155,7 +155,7 @@ public function getEnabledDefinitions(EditorInterface $editor): array {
   /**
    * {@inheritdoc}
    */
-  public function findPluginSupportingElement(string $tag) : ?string {
+  public function findPluginSupportingElement(string $tag): ?string {
     // This will contain the element config for a plugin found to support $tag,
     // so it can be compared to additional plugins that support $tag so the
     // plugin with the most permissive config can be the id returned.
@@ -237,7 +237,7 @@ public function getCKEditor5PluginConfig(EditorInterface $editor): array {
   /**
    * {@inheritdoc}
    */
-  public function getProvidedElements(array $plugin_ids = [], EditorInterface $editor = NULL, bool $retain_wildcard = FALSE) : array {
+  public function getProvidedElements(array $plugin_ids = [], EditorInterface $editor = NULL, bool $retain_wildcard = FALSE): array {
     $plugins = $this->getDefinitions();
     if (!empty($plugin_ids)) {
       $plugins = array_intersect_key($plugins, array_flip($plugin_ids));
@@ -356,7 +356,7 @@ protected function mergeDefinitionValues(string $get_method, array $definitions)
    * @return bool
    *   Whether the plugin is disabled due to unmet conditions.
    */
-  protected function isPluginDisabled(CKEditor5PluginInterface $plugin, EditorInterface $editor) : bool {
+  protected function isPluginDisabled(CKEditor5PluginInterface $plugin, EditorInterface $editor): bool {
     assert($plugin->getPluginDefinition()->hasConditions());
     foreach ($plugin->getPluginDefinition()->getConditions() as $condition_type => $required_value) {
       switch ($condition_type) {
diff --git a/core/modules/ckeditor5/src/Plugin/Editor/CKEditor5.php b/core/modules/ckeditor5/src/Plugin/Editor/CKEditor5.php
index 07a7391894..29504a45c0 100644
--- a/core/modules/ckeditor5/src/Plugin/Editor/CKEditor5.php
+++ b/core/modules/ckeditor5/src/Plugin/Editor/CKEditor5.php
@@ -183,7 +183,7 @@ public function settingsForm(array $form, FormStateInterface $form_state, Editor
    * @see \Drupal\editor\EditorInterface::getFilterFormat()
    * @see ckeditor5.pair.schema.yml
    */
-  public static function validatePair(EditorInterface $text_editor, FilterFormatInterface $text_format, bool $all_compatibility_problems = TRUE) : ConstraintViolationListInterface {
+  public static function validatePair(EditorInterface $text_editor, FilterFormatInterface $text_format, bool $all_compatibility_problems = TRUE): ConstraintViolationListInterface {
     if ($text_editor->getEditor() !== 'ckeditor5') {
       throw new \DomainException('This text editor is not configured to use CKEditor 5.');
     }
@@ -250,10 +250,11 @@ public function buildConfigurationForm(array $form, FormStateInterface $form_sta
     if ($editor->isNew() && !$form_state->get('ckeditor5_is_active') && $form_state->get('ckeditor5_is_selected')) {
       assert($editor->getSettings() === $this->getDefaultSettings());
       if (!$format->isNew()) {
-        $editor = $this->smartDefaultSettings->computeSmartDefaultSettings($editor, $format);
+        [$editor, $messages] = $this->smartDefaultSettings->computeSmartDefaultSettings($editor, $format);
+        foreach ($messages as $message) {
+          $this->messenger()->addMessage($message);
+        }
       }
-      // @todo In https://www.drupal.org/project/ckeditor5/issues/3226694 this
-      //   should ensure the appropriate messages get printed.
       $eventual_editor_and_format = $this->getEventualEditorWithPrimedFilterFormat($form_state, $editor);
       // Provide the validated eventual pair in form state to
       // ::getGeneratedAllowedHtmlValue(), to update filter_html's
@@ -715,7 +716,7 @@ protected static function createEphemeralPairedEditor(EditorInterface $editor, F
    * @return string
    *   The corresponding form name in the subform.
    */
-  protected static function mapViolationPropertyPathsToFormNames(string $property_path, array $subform) : string {
+  protected static function mapViolationPropertyPathsToFormNames(string $property_path, array $subform): string {
     $parts = explode('.', $property_path);
     // The "settings" form element does exist, but one level above the Text
     // Editor-specific form. This is operating on a subform.
@@ -745,7 +746,7 @@ protected static function mapViolationPropertyPathsToFormNames(string $property_
    * @return string
    *   The corresponding form name in the complete form.
    */
-  protected static function mapPairViolationPropertyPathsToFormNames(string $property_path, array $form) : string {
+  protected static function mapPairViolationPropertyPathsToFormNames(string $property_path, array $form): string {
     // Fundamental compatibility errors are at the root. Map these to the text
     // editor plugin dropdown.
     if ($property_path === '') {
diff --git a/core/modules/ckeditor5/src/Plugin/Validation/Constraint/EnabledConfigurablePluginsConstraintValidator.php b/core/modules/ckeditor5/src/Plugin/Validation/Constraint/EnabledConfigurablePluginsConstraintValidator.php
index 4edc561533..d59a669500 100644
--- a/core/modules/ckeditor5/src/Plugin/Validation/Constraint/EnabledConfigurablePluginsConstraintValidator.php
+++ b/core/modules/ckeditor5/src/Plugin/Validation/Constraint/EnabledConfigurablePluginsConstraintValidator.php
@@ -60,7 +60,7 @@ public function validate($settings, Constraint $constraint) {
    * @return \Drupal\ckeditor5\Plugin\CKEditor5PluginDefinition[]
    *   An array of enabled configurable CKEditor 5 plugin definitions.
    */
-  private function getConfigurableEnabledDefinitions() : array {
+  private function getConfigurableEnabledDefinitions(): array {
     $text_editor = $this->createTextEditorObjectFromContext();
     $enabled_definitions = $this->pluginManager->getEnabledDefinitions($text_editor);
     $configurable_enabled_definitions = array_filter($enabled_definitions, function (CKEditor5PluginDefinition $definition): bool {
diff --git a/core/modules/ckeditor5/src/Plugin/Validation/Constraint/FundamentalCompatibilityConstraintValidator.php b/core/modules/ckeditor5/src/Plugin/Validation/Constraint/FundamentalCompatibilityConstraintValidator.php
index 394153ce69..954f93e04d 100644
--- a/core/modules/ckeditor5/src/Plugin/Validation/Constraint/FundamentalCompatibilityConstraintValidator.php
+++ b/core/modules/ckeditor5/src/Plugin/Validation/Constraint/FundamentalCompatibilityConstraintValidator.php
@@ -74,7 +74,7 @@ public function validate($toolbar_item, Constraint $constraint) {
    * @param \Drupal\ckeditor5\Plugin\Validation\Constraint\FundamentalCompatibilityConstraint $constraint
    *   The constraint to validate.
    */
-  private function checkNoMarkupFilters(FilterFormatInterface $text_format, FundamentalCompatibilityConstraint $constraint) : void {
+  private function checkNoMarkupFilters(FilterFormatInterface $text_format, FundamentalCompatibilityConstraint $constraint): void {
     $markup_filters = static::getFiltersInFormatOfType(
       $text_format,
       FilterInterface::TYPE_MARKUP_LANGUAGE
@@ -97,7 +97,7 @@ private function checkNoMarkupFilters(FilterFormatInterface $text_format, Fundam
    * @param \Drupal\ckeditor5\Plugin\Validation\Constraint\FundamentalCompatibilityConstraint $constraint
    *   The constraint to validate.
    */
-  private function checkHtmlRestrictionsAreCompatible(FilterFormatInterface $text_format, FundamentalCompatibilityConstraint $constraint) : void {
+  private function checkHtmlRestrictionsAreCompatible(FilterFormatInterface $text_format, FundamentalCompatibilityConstraint $constraint): void {
     $minimum_tags = array_keys($this->pluginManager->getProvidedElements(self::FUNDAMENTAL_CKEDITOR5_PLUGINS));
 
     $html_restrictions = $text_format->getHtmlRestrictions();
@@ -133,7 +133,7 @@ private function checkHtmlRestrictionsAreCompatible(FilterFormatInterface $text_
    * @param \Drupal\ckeditor5\Plugin\Validation\Constraint\FundamentalCompatibilityConstraint $constraint
    *   The constraint to validate.
    */
-  private function checkHtmlRestrictionsMatch(EditorInterface $text_editor, FundamentalCompatibilityConstraint $constraint) : void {
+  private function checkHtmlRestrictionsMatch(EditorInterface $text_editor, FundamentalCompatibilityConstraint $constraint): void {
     $html_restrictor_filters = static::getFiltersInFormatOfType(
       $text_editor->getFilterFormat(),
       FilterInterface::TYPE_HTML_RESTRICTOR
@@ -192,7 +192,7 @@ private function checkHtmlRestrictionsMatch(EditorInterface $text_editor, Fundam
    * @return \Drupal\filter\Plugin\FilterInterface[]
    *   The matched filter plugins.
    */
-  private static function getFiltersInFormatOfType(FilterFormatInterface $text_format, int $filter_type, callable $extra_requirements = NULL) : array {
+  private static function getFiltersInFormatOfType(FilterFormatInterface $text_format, int $filter_type, callable $extra_requirements = NULL): array {
     assert(in_array($filter_type, [
       FilterInterface::TYPE_MARKUP_LANGUAGE,
       FilterInterface::TYPE_HTML_RESTRICTOR,
@@ -223,7 +223,7 @@ private static function getFiltersInFormatOfType(FilterFormatInterface $text_for
    *
    * @throws \InvalidArgumentException
    */
-  private static function findHtmlRestrictorFilterForbiddingTags(FilterFormatInterface $text_format, array $required_tags) : FilterInterface {
+  private static function findHtmlRestrictorFilterForbiddingTags(FilterFormatInterface $text_format, array $required_tags): FilterInterface {
     // Get HTML restrictor filters that actually restrict HTML.
     $filters = static::getFiltersInFormatOfType(
       $text_format,
@@ -264,7 +264,7 @@ function (FilterInterface $filter) {
    *
    * @throws \InvalidArgumentException
    */
-  private static function findHtmlRestrictorFilterNotAllowingTags(FilterFormatInterface $text_format, array $required_tags) : FilterInterface {
+  private static function findHtmlRestrictorFilterNotAllowingTags(FilterFormatInterface $text_format, array $required_tags): FilterInterface {
     // Get HTML restrictor filters that actually restrict HTML.
     $filters = static::getFiltersInFormatOfType(
       $text_format,
diff --git a/core/modules/ckeditor5/src/Plugin/Validation/Constraint/TextEditorObjectDependentValidatorTrait.php b/core/modules/ckeditor5/src/Plugin/Validation/Constraint/TextEditorObjectDependentValidatorTrait.php
index baf51cc900..63bf2f0dad 100644
--- a/core/modules/ckeditor5/src/Plugin/Validation/Constraint/TextEditorObjectDependentValidatorTrait.php
+++ b/core/modules/ckeditor5/src/Plugin/Validation/Constraint/TextEditorObjectDependentValidatorTrait.php
@@ -22,7 +22,7 @@ trait TextEditorObjectDependentValidatorTrait {
    * @return \Drupal\editor\EditorInterface
    *   A text editor object, with the text format pre-populated.
    */
-  private function createTextEditorObjectFromContext() : EditorInterface {
+  private function createTextEditorObjectFromContext(): EditorInterface {
     if ($this->context->getRoot()->getDataDefinition()->getDataType() === 'ckeditor5_valid_pair__format_and_editor') {
       $text_format = FilterFormat::create([
         'filters' => $this->context->getRoot()->get('filters')->toArray(),
diff --git a/core/modules/ckeditor5/src/Plugin/Validation/Constraint/ToolbarItemConstraintValidator.php b/core/modules/ckeditor5/src/Plugin/Validation/Constraint/ToolbarItemConstraintValidator.php
index f75f9c9204..ea56a6a050 100644
--- a/core/modules/ckeditor5/src/Plugin/Validation/Constraint/ToolbarItemConstraintValidator.php
+++ b/core/modules/ckeditor5/src/Plugin/Validation/Constraint/ToolbarItemConstraintValidator.php
@@ -45,7 +45,7 @@ public function validate($toolbar_item, Constraint $constraint) {
    * @return bool
    *   Whether the given toolbar item is valid or not.
    */
-  protected function isValidToolbarItem(string $toolbar_item) : bool {
+  protected function isValidToolbarItem(string $toolbar_item): bool {
     // Special case: the toolbar group separator.
     // @see https://ckeditor.com/docs/ckeditor5/latest/features/toolbar/toolbar.html#separating-toolbar-items
     if ($toolbar_item === '|') {
diff --git a/core/modules/ckeditor5/src/SmartDefaultSettings.php b/core/modules/ckeditor5/src/SmartDefaultSettings.php
index c844b8d5db..42e10b8740 100644
--- a/core/modules/ckeditor5/src/SmartDefaultSettings.php
+++ b/core/modules/ckeditor5/src/SmartDefaultSettings.php
@@ -4,9 +4,12 @@
 
 namespace Drupal\ckeditor5;
 
+use Drupal\ckeditor\CKEditorPluginButtonsInterface;
+use Drupal\ckeditor\CKEditorPluginContextualInterface;
+use Drupal\ckeditor\CKEditorPluginManager;
 use Drupal\ckeditor5\Plugin\CKEditor5PluginDefinition;
+use Drupal\ckeditor5\Plugin\CKEditor5PluginManagerInterface;
 use Drupal\Component\Plugin\PluginManagerInterface;
-use Drupal\Core\Messenger\MessengerTrait;
 use Drupal\Core\StringTranslation\StringTranslationTrait;
 use Drupal\editor\EditorInterface;
 use Drupal\editor\Entity\Editor;
@@ -20,24 +23,43 @@
  */
 final class SmartDefaultSettings {
 
-  use MessengerTrait;
   use StringTranslationTrait;
 
   /**
    * The CKEditor 5 plugin manager.
    *
-   * @var \Drupal\Component\Plugin\PluginManagerInterface
+   * @var \Drupal\ckeditor5\Plugin\CKEditor5PluginManagerInterface
    */
   protected $pluginManager;
 
+  /**
+   * The CKEditor 4 to 5 upgrade plugin manager.
+   *
+   * @var \Drupal\Component\Plugin\PluginManagerInterface
+   */
+  protected $upgradePluginManager;
+
+  /**
+   * The "CKEditor 4 plugin" plugin manager.
+   *
+   * @var \Drupal\ckeditor\CKEditorPluginManager
+   */
+  protected $cke4PluginManager;
+
   /**
    * Constructs a SmartDefaultSettings object.
    *
-   * @param \Drupal\Component\Plugin\PluginManagerInterface $plugin_manager
+   * @param \Drupal\ckeditor5\Plugin\CKEditor5PluginManagerInterface $plugin_manager
    *   The CKEditor 5 plugin manager.
+   * @param \Drupal\Component\Plugin\PluginManagerInterface $upgrade_plugin_manager
+   *   The CKEditor 4 to 5 upgrade plugin manager.
+   * @param \Drupal\ckeditor\CKEditorPluginManager $cke4_plugin_manager
+   *   The CKEditor 4 plugin manager.
    */
-  public function __construct(PluginManagerInterface $plugin_manager) {
+  public function __construct(CKEditor5PluginManagerInterface $plugin_manager, PluginManagerInterface $upgrade_plugin_manager, CKEditorPluginManager $cke4_plugin_manager = NULL) {
     $this->pluginManager = $plugin_manager;
+    $this->upgradePluginManager = $upgrade_plugin_manager;
+    $this->cke4PluginManager = $cke4_plugin_manager;
   }
 
   /**
@@ -49,11 +71,13 @@ public function __construct(PluginManagerInterface $plugin_manager) {
    * @param \Drupal\filter\FilterFormatInterface $text_format
    *   The text format for which to compute smart default settings.
    *
-   * @return \Drupal\editor\EditorInterface
-   *   The cloned config entity with settings modified for CKEditor 5 … or a
-   *   completely new config entity if this text format did not yet have one.
+   * @return array
+   *   An array with two values:
+   *   1. The cloned config entity with settings modified for CKEditor 5 … or a
+   *      completely new config entity if this text format did not yet have one.
+   *   2. Messages explaining what conclusions were reached.
    */
-  public function computeSmartDefaultSettings(?EditorInterface $text_editor, FilterFormatInterface $text_format) : EditorInterface {
+  public function computeSmartDefaultSettings(?EditorInterface $text_editor, FilterFormatInterface $text_format): array {
     if ($text_format->isNew()) {
       throw new \InvalidArgumentException('Smart default settings can only be computed when there is a pre-existing text format.');
     }
@@ -61,6 +85,8 @@ public function computeSmartDefaultSettings(?EditorInterface $text_editor, Filte
       throw new \InvalidArgumentException('The given text editor and text format must form a pair.');
     }
 
+    $messages = [];
+
     // Ensure that unsaved changes to the text format object are also respected.
     if ($text_editor) {
       // Overwrite the Editor config entity object's $filterFormat property, to
@@ -92,7 +118,8 @@ public function computeSmartDefaultSettings(?EditorInterface $text_editor, Filte
     // if it exists.
     $old_editor = $editor->id() ? Editor::load($editor->id()) : NULL;
     if ($old_editor && $old_editor->getEditor() === 'ckeditor') {
-      $upgraded_settings = $this->createSettingsFromCKEditor4($old_editor->getSettings());
+      $enabled_cke4_plugins = $this->getEnabledCkeditor4Plugins($old_editor);
+      [$upgraded_settings, $messages] = $this->createSettingsFromCKEditor4($old_editor->getSettings(), $enabled_cke4_plugins);
       $editor->setSettings($upgraded_settings);
       $editor->setImageUploadSettings($old_editor->getImageUploadSettings());
     }
@@ -103,19 +130,18 @@ public function computeSmartDefaultSettings(?EditorInterface $text_editor, Filte
     if ($result !== NULL) {
       [$enabling_message_content, $unsupported] = $result;
       if ($enabling_message_content) {
-        $enabling_message = $this->t('The following plugins were enabled to support tags that are allowed by this text format: %enabling_message_content.',
+        $messages[] = $this->t('The following plugins were enabled to support tags that are allowed by this text format: %enabling_message_content.',
           ['%enabling_message_content' => $enabling_message_content],
         );
-        $this->messenger()->addMessage($enabling_message);
       }
       unset($unsupported['*']);
       // Warn user about unsupported tags.
       if (!empty($unsupported)) {
         $unsupported_string = implode(' ', HTMLRestrictionsUtilities::toReadableElements($unsupported));
         $this->addTagsToSourceEditing($editor, $unsupported_string);
-        $this->messenger()->addMessage($this->t("The following tags were permitted by this format's filter configuration, but no plugin was available that supports them. To ensure the tags remain supported by this text format, the following were added to the Source Editing plugin's <em>Manually editable HTML tags</em>: @unsupported_string.", [
+        $messages[] = $this->t("The following tags were permitted by this format's filter configuration, but no plugin was available that supports them. To ensure the tags remain supported by this text format, the following were added to the Source Editing plugin's <em>Manually editable HTML tags</em>: @unsupported_string.", [
           '@unsupported_string' => $unsupported_string,
-        ]));
+        ]);
       }
     }
 
@@ -125,17 +151,16 @@ public function computeSmartDefaultSettings(?EditorInterface $text_editor, Filte
     if ($result !== NULL) {
       [$enabled_for_attributes_message_content, $missing_attributes] = $result;
       if ($enabled_for_attributes_message_content) {
-        $enabled_for_attributes_message = $this->t('The following plugins were enabled to support specific attributes that are allowed by this text format: %enabled_for_attributes_message_content.',
+        $messages[] = $this->t('The following plugins were enabled to support specific attributes that are allowed by this text format: %enabled_for_attributes_message_content.',
           ['%enabled_for_attributes_message_content' => $enabled_for_attributes_message_content],
         );
-        $this->messenger()->addMessage($enabled_for_attributes_message);
       }
       // Warn user about supported tags but missing attributes.
       if ($missing_attributes) {
         $this->addTagsToSourceEditing($editor, $missing_attributes);
-        $this->messenger()->addMessage($this->t("This format's HTML filters includes plugins that support the following tags, but not some of their attributes. To ensure these attributes remain supported by this text format, the following were added to the Source Editing plugin's <em>Manually editable HTML tags</em>: @missing_attributes.", [
+        $messages[] = $this->t("This format's HTML filters includes plugins that support the following tags, but not some of their attributes. To ensure these attributes remain supported by this text format, the following were added to the Source Editing plugin's <em>Manually editable HTML tags</em>: @missing_attributes.", [
           '@missing_attributes' => $missing_attributes,
-        ]));
+        ]);
       }
     }
 
@@ -143,19 +168,20 @@ public function computeSmartDefaultSettings(?EditorInterface $text_editor, Filte
     // and add their default settings.
     $this->addDefaultSettingsForEnabledConfigurablePlugins($editor);
 
-    return $editor;
+    return [$editor, $messages];
   }
 
-  private function addTagsToSourceEditing(EditorInterface $editor, $tags) : void {
+  private function addTagsToSourceEditing(EditorInterface $editor, string $tags): array {
+    $messages = [];
     $settings = $editor->getSettings();
     if (!isset($settings['toolbar']['items']) || !in_array('sourceEditing', $settings['toolbar']['items'])) {
-      $enabling_message = $this->t('The <em>Source Editing</em> plugin was enabled to support tags and/or attributes that are not explicitly supported by any available CKEditor 5 plugins.');
-      $this->messenger()->addMessage($enabling_message);
+      $messages[] = $this->t('The <em>Source Editing</em> plugin was enabled to support tags and/or attributes that are not explicitly supported by any available CKEditor 5 plugins.');
       $settings['toolbar']['items'][] = 'sourceEditing';
     }
     $source_editing_allowed_tags = $settings['plugins']['ckeditor5_sourceEditing']['allowed_tags'] ?? [];
     $settings['plugins']['ckeditor5_sourceEditing']['allowed_tags'] = array_merge($source_editing_allowed_tags, HTMLRestrictionsUtilities::allowedElementsStringToPluginElementsArray($tags));
     $editor->setSettings($settings);
+    return $messages;
   }
 
   /**
@@ -164,12 +190,23 @@ private function addTagsToSourceEditing(EditorInterface $editor, $tags) : void {
    * @param array $ckeditor4_settings
    *   The value for "settings" in a Text Editor config entity configured to use
    *   CKEditor 4.
+   * @param string[] $enabled_ckeditor4_plugins
+   *   The list of enabled CKEditor 4 plugins: their settings will be mapped to
+   *   the CKEditor 5 equivalents, if they have any.
    *
    * @return array
-   *   An equivalent value for CKEditor 5.
+   *   An array with two values:
+   *   1. An equivalent value for CKEditor 5.
+   *   2. Messages explaining upgrade path issues.
    */
-  private static function createSettingsFromCKEditor4(array $ckeditor4_settings) : array {
-    $settings = [];
+  private function createSettingsFromCKEditor4(array $ckeditor4_settings, array $enabled_ckeditor4_plugins): array {
+    $settings = [
+      'toolbar' => [
+        'items' => [],
+      ],
+      'plugins' => [],
+    ];
+    $messages = [];
 
     // First: toolbar items.
     // @see \Drupal\ckeditor\CKEditorPluginButtonsInterface
@@ -177,7 +214,15 @@ private static function createSettingsFromCKEditor4(array $ckeditor4_settings) :
       foreach ($row as $group) {
         $some_added = FALSE;
         foreach ($group['items'] as $cke4_button) {
-          $equivalent = static::mapCKEditor4ToolbarButtonToCKEditor5ToolbarItem($cke4_button);
+          try {
+            $equivalent = $this->upgradePluginManager->mapCKEditor4ToolbarButtonToCKEditor5ToolbarItem($cke4_button);
+          }
+          catch (\OutOfBoundsException $e) {
+            $messages[] = $this->t('The CKEditor 4 button %button does not have a known upgrade path. If it allowed editing markup, then you can do so now through the Source Editing functionality.', [
+              '%button' => $cke4_button,
+            ]);
+            continue;
+          }
           if ($equivalent) {
             $settings['toolbar']['items'][] = $equivalent;
             $some_added = TRUE;
@@ -196,141 +241,74 @@ private static function createSettingsFromCKEditor4(array $ckeditor4_settings) :
 
     // Second: plugin settings.
     // @see \Drupal\ckeditor\CKEditorPluginConfigurableInterface
-    // @todo Generalize in https://www.drupal.org/i/3226335.
-    foreach ($ckeditor4_settings['plugins'] as $cke4_plugin_id => $setting) {
-      switch ($cke4_plugin_id) {
-        // @see \Drupal\ckeditor\Plugin\CKEditorPlugin\StylesCombo
-        case 'stylescombo':
-          // No equivalent exists yet.
-          break;
-
-        // @see \Drupal\ckeditor\Plugin\CKEditorPlugin\Language
-        case 'language':
-          // In CKEditor 4, the language plugin is always part of settings, even
-          // if the language button isn't actually in use. Only add the plugin
-          // to CKEditor 5 if the language button is in the toolbar.
-          $language_in_toolbar = FALSE;
-          array_walk_recursive($ckeditor4_settings['toolbar'], function ($value) use (&$language_in_toolbar) {
-            if ($value === 'Language') {
-              $language_in_toolbar = TRUE;
-            }
-          });
-          if ($language_in_toolbar) {
-            $settings['plugins']['language'] = $setting;
-          }
-          break;
+    $enabled_ckeditor4_plugins_with_settings = array_intersect_key($ckeditor4_settings['plugins'], array_flip($enabled_ckeditor4_plugins));
+    foreach ($enabled_ckeditor4_plugins_with_settings as $cke4_plugin_id => $cke4_plugin_settings) {
+      try {
+        $cke5_plugin_settings = $this->upgradePluginManager->mapCKEditor4SettingsToCKEditor5Configuration($cke4_plugin_id, $cke4_plugin_settings);
+        assert(count($cke5_plugin_settings) === 1);
+        $cke5_plugin_id = array_keys($cke5_plugin_settings)[0];
+        if (isset($settings['plugins'][$cke5_plugin_id])) {
+          throw new \LogicException(sprintf('The %s plugin settings have already been upgraded. Only a single @CKEditor4To5Upgrade is allowed to migrate the settings for a particular CKEditor 4 plugin.', $cke5_plugin_id));
+        }
+        $settings['plugins'] += $cke5_plugin_settings;
+      }
+      catch (\OutOfBoundsException $e) {
+        $messages[] = $this->t('The %cke4_plugin_id plugin settings do not have a known upgrade path.', [
+          '%cke4_plugin_id' => $cke4_plugin_id,
+        ]);
+        continue;
       }
     }
 
-    return $settings;
+    return [$settings, $messages];
   }
 
   /**
-   * Maps a CKEditor 4 button to the CKEditor 5 equivalent, if it exists.
+   * Gets all enabled CKEditor 4 plugins.
    *
-   * Generated by inspecting all \Drupal\ckeditor\CKEditorPluginButtonsInterface
-   * implementations.
-   *
-   * @param string $cke4_button
-   *   A valid CKEditor 4 button name.
-   *
-   * @return string|null
-   *   The equivalent CKEditor 5 toolbar item, or NULL if no equivalent exists.
+   * @param \Drupal\editor\EditorInterface $editor
+   *   A text editor config entity configured to use CKEditor 4.
    *
-   * @see \Drupal\ckeditor\CKEditorPluginButtonsInterface
-   * @todo Generalize in https://www.drupal.org/i/3226335.
+   * @return string[]
+   *   The enabled CKEditor 4 plugin IDs.
    */
-  private static function mapCKEditor4ToolbarButtonToCKEditor5ToolbarItem(string $cke4_button) : ?string {
-    switch ($cke4_button) {
-      // @see \Drupal\ckeditor\Plugin\CKEditorPlugin\DrupalImage
-      case 'DrupalImage':
-        return 'uploadImage';
-
-      // @see \Drupal\ckeditor\Plugin\CKEditorPlugin\DrupalLink
-      case 'DrupalLink':
-        return 'link';
-
-      case 'DrupalUnlink':
-        return NULL;
-
-      // @see \Drupal\ckeditor\Plugin\CKEditorPlugin\Internal
-      case 'Bold':
-      case 'Italic':
-      case 'Underline':
-      case 'Superscript':
-      case 'Subscript':
-      case 'BulletedList':
-      case 'NumberedList':
-      case 'Outdent':
-      case 'Indent':
-      case 'Undo':
-      case 'Redo':
-        return lcfirst($cke4_button);
-
-      case 'Blockquote':
-        return 'blockQuote';
-
-      case 'JustifyLeft':
-        return "alignment:left";
-
-      case 'JustifyCenter':
-        return "alignment:center";
-
-      case 'JustifyRight':
-        return "alignment:right";
-
-      case 'JustifyBlock':
-        return "alignment:justify";
-
-      case 'HorizontalRule':
-        return 'horizontalLine';
-
-      case 'Format':
-        return 'heading';
-
-      case 'Table':
-        return 'insertTable';
-
-      case 'Source':
-        return 'sourceEditing';
-
-      case 'Strike':
-        return 'strikethrough';
-
-      case 'Cut':
-      case 'Copy':
-      case 'Paste':
-      case 'PasteText':
-      case 'PasteFromWord':
-      case 'ShowBlocks':
-      case 'Maximize':
-      case '-':
-        // @see https://www.drupal.org/project/ckeditor5/issues/3211049#comment-14167764
-        return NULL;
-
-      // @see \Drupal\ckeditor5\Plugin\CKEditor5Plugin\RemoveFormat
-      case 'RemoveFormat':
-        return 'removeFormat';
-
-      // @see \Drupal\ckeditor\Plugin\CKEditorPlugin\StylesCombo
-      case 'Styles':
-        return NULL;
-
-      // @see \Drupal\ckeditor5\Plugin\CKEditor5Plugin\specialCharacters
-      case 'SpecialChar':
-        return 'specialCharacters';
-
-      // @see \Drupal\ckeditor\Plugin\CKEditorPlugin\Language
-      case 'Language':
-        return 'textPartLanguage';
-
-      // @see \Drupal\media_library\Plugin\CKEditorPlugin\DrupalMediaLibrary
-      case 'DrupalMediaLibrary':
-        return 'drupalMedia';
-
-      default:
-        return NULL;
+  protected function getEnabledCkeditor4Plugins(EditorInterface $editor): array {
+    assert($editor->getEditor() === 'ckeditor');
+
+    // This is largely copied from the CKEditor 4 plugin manager, because it
+    // unfortunately does not provide the API this needs.
+    // @see \Drupal\ckeditor\CKEditorPluginManager::getEnabledPluginFiles()
+    $plugins = array_keys($this->cke4PluginManager->getDefinitions());
+    $toolbar_buttons = $this->cke4PluginManager->getEnabledButtons($editor);
+    $enabled_plugins = [];
+    $additional_plugins = [];
+    foreach ($plugins as $plugin_id) {
+      $plugin = $this->cke4PluginManager->createInstance($plugin_id);
+
+      $enabled = FALSE;
+      // Enable this plugin if it provides a button that has been enabled.
+      if ($plugin instanceof CKEditorPluginButtonsInterface) {
+        $plugin_buttons = array_keys($plugin->getButtons());
+        $enabled = (count(array_intersect($toolbar_buttons, $plugin_buttons)) > 0);
+      }
+      // Otherwise enable this plugin if it declares itself as enabled.
+      if (!$enabled && $plugin instanceof CKEditorPluginContextualInterface) {
+        $enabled = $plugin->isEnabled($editor);
+      }
+
+      if ($enabled) {
+        $enabled_plugins[] = $plugin_id;
+        // Check if this plugin has dependencies that also need to be enabled.
+        $additional_plugins = array_merge($additional_plugins, array_diff($plugin->getDependencies($editor), $additional_plugins));
+      }
     }
+
+    // Add the list of dependent plugins.
+    foreach ($additional_plugins as $plugin_id) {
+      $enabled_plugins[$plugin_id] = $plugin_id;
+    }
+
+    return $enabled_plugins;
   }
 
   /**
@@ -347,7 +325,7 @@ private static function mapCKEditor4ToolbarButtonToCKEditor5ToolbarItem(string $
    *      enabled to match the HTML tags allowed by the text format.
    *   2. the unsupported tags
    */
-  private function addToolbarItemsToMatchHtmlTagsInFormat(FilterFormatInterface $format, EditorInterface $editor) : ?array {
+  private function addToolbarItemsToMatchHtmlTagsInFormat(FilterFormatInterface $format, EditorInterface $editor): ?array {
     $html_restrictions_needed_elements = $format->getHtmlRestrictions();
     if ($html_restrictions_needed_elements === FALSE) {
       return NULL;
@@ -427,7 +405,7 @@ private function addToolbarItemsToMatchHtmlTagsInFormat(FilterFormatInterface $f
    *      enabled to match the HTML attributes allowed by the text format.
    *   2. the unsupported attributes
    */
-  private function addToolbarItemsToMatchHtmlAttributesInFormat(FilterFormatInterface $format, EditorInterface $editor) : ?array {
+  private function addToolbarItemsToMatchHtmlAttributesInFormat(FilterFormatInterface $format, EditorInterface $editor): ?array {
     $html_restrictions_needed_elements = $format->getHtmlRestrictions();
     if ($html_restrictions_needed_elements === FALSE) {
       return NULL;
@@ -597,7 +575,7 @@ private function addToolbarItemsToMatchHtmlAttributesInFormat(FilterFormatInterf
    * @param \Drupal\editor\EditorInterface $editor
    *   The text editor config entity to update.
    */
-  private function addDefaultSettingsForEnabledConfigurablePlugins(EditorInterface $editor) : void {
+  private function addDefaultSettingsForEnabledConfigurablePlugins(EditorInterface $editor): void {
     $settings = $editor->getSettings();
     $update_settings = FALSE;
     $enabled_definitions = $this->pluginManager->getEnabledDefinitions($editor);
diff --git a/core/modules/ckeditor5/tests/modules/ckeditor4to5upgrade_plugin_test/ckeditor4to5upgrade_plugin_test.info.yml b/core/modules/ckeditor5/tests/modules/ckeditor4to5upgrade_plugin_test/ckeditor4to5upgrade_plugin_test.info.yml
new file mode 100644
index 0000000000..d86e9bb5b9
--- /dev/null
+++ b/core/modules/ckeditor5/tests/modules/ckeditor4to5upgrade_plugin_test/ckeditor4to5upgrade_plugin_test.info.yml
@@ -0,0 +1,7 @@
+name: CKEditor 4 to 5 Upgrade plugin Test
+type: module
+description: "Provides test plugins for testing CKEditor 4 to 5 upgrade infrastructure."
+package: Testing
+core_version_requirement: ^9
+dependencies:
+  - ckeditor5:ckeditor5
diff --git a/core/modules/ckeditor5/tests/modules/ckeditor4to5upgrade_plugin_test/ckeditor4to5upgrade_plugin_test.module b/core/modules/ckeditor5/tests/modules/ckeditor4to5upgrade_plugin_test/ckeditor4to5upgrade_plugin_test.module
new file mode 100644
index 0000000000..2479bf3173
--- /dev/null
+++ b/core/modules/ckeditor5/tests/modules/ckeditor4to5upgrade_plugin_test/ckeditor4to5upgrade_plugin_test.module
@@ -0,0 +1,40 @@
+<?php
+
+/**
+ * @file
+ * Implements hooks for the CKEditor 4 to 5 Upgrade plugin Test module.
+ */
+
+declare(strict_types = 1);
+
+/**
+ * Implements hook_ckeditor4to5upgrade_plugin_info_alter().
+ */
+function ckeditor4to5upgrade_plugin_test_ckeditor4to5upgrade_plugin_info_alter(array &$plugin_definitions): void {
+  switch (\Drupal::state()->get('ckeditor4to5upgrade_plugin_test')) {
+    case 'duplicate_button':
+      $plugin_definitions['foo'] = array_intersect_key($plugin_definitions['core'], ['cke4_buttons' => TRUE]);
+      break;
+
+    case 'duplicate_plugin_settings':
+      $plugin_definitions['foo'] = array_intersect_key($plugin_definitions['core'], ['cke4_plugin_settings' => TRUE]);
+      break;
+
+    case 'lying_button':
+      $plugin_definitions['foo'] = [
+        'cke4_buttons' => ['foo'],
+        'class' => $plugin_definitions['core']['class'],
+      ];
+      break;
+
+    case 'lying_plugin_settings':
+      $plugin_definitions['foo'] = [
+        'cke4_plugin_settings' => ['foo'],
+        'class' => $plugin_definitions['core']['class'],
+      ];
+      break;
+
+    default:
+      throw new \LogicException();
+  }
+}
diff --git a/core/modules/ckeditor5/tests/src/FunctionalJavascript/QuickEditIntegrationTest.php b/core/modules/ckeditor5/tests/src/FunctionalJavascript/QuickEditIntegrationTest.php
index 4f6561dc58..5561485194 100644
--- a/core/modules/ckeditor5/tests/src/FunctionalJavascript/QuickEditIntegrationTest.php
+++ b/core/modules/ckeditor5/tests/src/FunctionalJavascript/QuickEditIntegrationTest.php
@@ -130,8 +130,6 @@ public function testDiscard() {
     ]);
     $this->assertEntityInstanceFieldStates('node', 1, 0, [
       'node/1/title/en/full' => 'inactive',
-      'node/1/uid/en/full' => 'inactive',
-      'node/1/created/en/full' => 'inactive',
       'node/1/body/en/full' => 'inactive',
     ]);
 
@@ -143,8 +141,6 @@ public function testDiscard() {
     $this->assertQuickEditEntityToolbar((string) $node->label(), NULL);
     $this->assertEntityInstanceFieldStates('node', 1, 0, [
       'node/1/title/en/full' => 'candidate',
-      'node/1/uid/en/full' => 'candidate',
-      'node/1/created/en/full' => 'candidate',
       'node/1/body/en/full' => 'candidate',
     ]);
 
@@ -157,8 +153,6 @@ public function testDiscard() {
     $this->assertQuickEditEntityToolbar((string) $node->label(), 'Body');
     $this->assertEntityInstanceFieldStates('node', 1, 0, [
       'node/1/title/en/full' => 'candidate',
-      'node/1/uid/en/full' => 'candidate',
-      'node/1/created/en/full' => 'candidate',
       'node/1/body/en/full' => 'active',
     ]);
     hold_test_response(FALSE);
@@ -176,8 +170,6 @@ public function testDiscard() {
     $assert_session->waitForElement('css', '.quickedit-toolbar-field div[id*="body"].quickedit-changed');
     $this->assertEntityInstanceFieldStates('node', 1, 0, [
       'node/1/title/en/full' => 'candidate',
-      'node/1/uid/en/full' => 'candidate',
-      'node/1/created/en/full' => 'candidate',
       'node/1/body/en/full' => 'changed',
     ]);
 
@@ -223,8 +215,6 @@ public function testArticleNode() {
     ]);
     $this->assertEntityInstanceFieldStates('node', 1, 0, [
       'node/1/title/en/full' => 'inactive',
-      'node/1/uid/en/full' => 'inactive',
-      'node/1/created/en/full' => 'inactive',
       'node/1/body/en/full' => 'inactive',
     ]);
 
@@ -236,8 +226,6 @@ public function testArticleNode() {
     $this->assertQuickEditEntityToolbar((string) $node->label(), NULL);
     $this->assertEntityInstanceFieldStates('node', 1, 0, [
       'node/1/title/en/full' => 'candidate',
-      'node/1/uid/en/full' => 'candidate',
-      'node/1/created/en/full' => 'candidate',
       'node/1/body/en/full' => 'candidate',
     ]);
 
@@ -248,8 +236,6 @@ public function testArticleNode() {
     $this->assertQuickEditEntityToolbar((string) $node->label(), 'Body');
     $this->assertEntityInstanceFieldStates('node', 1, 0, [
       'node/1/title/en/full' => 'candidate',
-      'node/1/uid/en/full' => 'candidate',
-      'node/1/created/en/full' => 'candidate',
       'node/1/body/en/full' => 'active',
     ]);
     hold_test_response(FALSE);
@@ -267,8 +253,6 @@ public function testArticleNode() {
     $assert_session->waitForElement('css', '.quickedit-toolbar-field div[id*="body"].quickedit-changed');
     $this->assertEntityInstanceFieldStates('node', 1, 0, [
       'node/1/title/en/full' => 'candidate',
-      'node/1/uid/en/full' => 'candidate',
-      'node/1/created/en/full' => 'candidate',
       'node/1/body/en/full' => 'changed',
     ]);
 
@@ -281,8 +265,6 @@ public function testArticleNode() {
 
     $this->assertEntityInstanceFieldStates('node', 1, 0, [
       'node/1/title/en/full' => 'candidate',
-      'node/1/uid/en/full' => 'candidate',
-      'node/1/created/en/full' => 'candidate',
       'node/1/body/en/full' => 'saving',
     ]);
 
diff --git a/core/modules/ckeditor5/tests/src/Kernel/CKEditor4to5UpgradeCompletenessTest.php b/core/modules/ckeditor5/tests/src/Kernel/CKEditor4to5UpgradeCompletenessTest.php
new file mode 100644
index 0000000000..26d10a663b
--- /dev/null
+++ b/core/modules/ckeditor5/tests/src/Kernel/CKEditor4to5UpgradeCompletenessTest.php
@@ -0,0 +1,193 @@
+<?php
+
+declare(strict_types = 1);
+
+namespace Drupal\Tests\ckeditor5\Kernel;
+
+use Drupal\ckeditor\CKEditorPluginConfigurableInterface;
+use Drupal\Component\Utility\NestedArray;
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * @covers \Drupal\ckeditor5\Plugin\CKEditor4To5Upgrade\Core
+ * @group ckeditor5
+ */
+class CKEditor4to5UpgradeCompletenessTest extends KernelTestBase {
+
+  /**
+   * The "CKEditor 4 plugin" plugin manager.
+   *
+   * @var \Drupal\ckeditor\CKEditorPluginManager
+   */
+  protected $cke4PluginManager;
+
+  /**
+   * The "CKEditor 5 plugin" plugin manager.
+   *
+   * @var \Drupal\ckeditor5\Plugin\CKEditor5PluginManagerInterface
+   */
+  protected $cke5PluginManager;
+
+  /**
+   * The CKEditor 4 to 5 upgrade plugin manager.
+   *
+   * @var \Drupal\ckeditor5\Plugin\CKEditor4To5UpgradePluginManager
+   */
+  protected $upgradePluginManager;
+
+  /**
+   * Smart default settings utility.
+   *
+   * @var \Drupal\ckeditor5\SmartDefaultSettings
+   */
+  protected $smartDefaultSettings;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'ckeditor',
+    'ckeditor5',
+    // Enabled because of \Drupal\media\Plugin\CKEditorPlugin\DrupalMedia.
+    'media',
+    // Enabled because of \Drupal\media_library\Plugin\CKEditorPlugin\DrupalMediaLibrary.
+    'media_library',
+    // Enabled for media_library.
+    'views',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+    $this->cke4PluginManager = $this->container->get('plugin.manager.ckeditor.plugin');
+    $this->cke5PluginManager = $this->container->get('plugin.manager.ckeditor5.plugin');
+    $this->upgradePluginManager = $this->container->get('plugin.manager.ckeditor4to5upgrade.plugin');
+    $this->smartDefaultSettings = $this->container->get('ckeditor5.smart_default_settings');
+  }
+
+  /**
+   * Tests that all CKEditor 4 buttons in core have an upgrade path.
+   */
+  public function testButtons(): void {
+    $cke4_buttons = array_keys(NestedArray::mergeDeepArray($this->cke4PluginManager->getButtons()));
+
+    foreach ($cke4_buttons as $button) {
+      $equivalent = $this->upgradePluginManager->mapCKEditor4ToolbarButtonToCKEditor5ToolbarItem($button);
+      $this->assertTrue($equivalent === NULL || is_string($equivalent));
+      // The returned equivalent CKEditor 5 toolbar item must exist.
+      if (is_string($equivalent)) {
+        $this->assertArrayHasKey($equivalent, $this->cke5PluginManager->getToolbarItems());
+      }
+    }
+  }
+
+  /**
+   * Tests that the test-only CKEditor 4 module does not have an upgrade path.
+   */
+  public function testButtonsWithTestOnlyModule(): void {
+    $this->enableModules(['ckeditor_test']);
+    $this->cke4PluginManager = $this->container->get('plugin.manager.ckeditor.plugin');
+
+    $this->expectException(\OutOfBoundsException::class);
+    $this->expectExceptionMessage('No upgrade path found for the "LlamaCSS" button.');
+
+    $this->testButtons();
+  }
+
+  /**
+   * Tests that all configurable CKEditor 4 plugins in core have an upgrade path.
+   */
+  public function testSettings(): void {
+    $cke4_configurable_plugins = [];
+    foreach ($this->cke4PluginManager->getDefinitions() as $plugin_id => $definition) {
+      // Special case: DrupalImage.
+      // @see \Drupal\ckeditor\Plugin\CKEditorPlugin\DrupalImage
+      // @see \Drupal\editor\Entity\Editor::getImageUploadSettings()
+      if ($plugin_id === 'drupalimage') {
+        continue;
+      }
+
+      if (is_subclass_of($definition['class'], CKEditorPluginConfigurableInterface::class)) {
+        $cke4_configurable_plugins[] = $plugin_id;
+      }
+    }
+
+    foreach ($cke4_configurable_plugins as $plugin_id) {
+      $cke5_plugin_settings = $this->upgradePluginManager->mapCKEditor4SettingsToCKEditor5Configuration($plugin_id, []);
+      $this->assertTrue($cke5_plugin_settings === NULL || is_array($cke5_plugin_settings));
+      // The returned equivalent CKEditor 5 plugin must exist.
+      if (is_array($cke5_plugin_settings)) {
+        $cke5_plugin_id = array_keys($cke5_plugin_settings)[0];
+        $this->assertArrayHasKey($cke5_plugin_id, $this->cke5PluginManager->getDefinitions());
+      }
+    }
+  }
+
+  /**
+   * Tests that the test-only CKEditor 4 module does not have an upgrade path.
+   */
+  public function testSettingsWithTestOnlyModule(): void {
+    $this->enableModules(['ckeditor_test']);
+    $this->cke4PluginManager = $this->container->get('plugin.manager.ckeditor.plugin');
+
+    $this->expectException(\OutOfBoundsException::class);
+    $this->expectExceptionMessage('No upgrade path found for the "llama_contextual_and_button" plugin settings.');
+
+    $this->testSettings();
+  }
+
+  /**
+   * Tests that only one plugin can provide an upgrade path for a button.
+   */
+  public function testOnlyOneUpgradePluginAllowedPerCKEditor4Button(): void {
+    $this->enableModules(['ckeditor4to5upgrade_plugin_test']);
+    \Drupal::state()->set('ckeditor4to5upgrade_plugin_test', 'duplicate_button');
+
+    $this->expectException(\OutOfBoundsException::class);
+    $this->expectExceptionMessage('The "DrupalImage" CKEditor 4 button is already being upgraded by the "core" CKEditor4To5Upgrade plugin, the "foo" plugin is as well. This conflict needs to be resolved.');
+
+    $this->upgradePluginManager->mapCKEditor4ToolbarButtonToCKEditor5ToolbarItem('foo');
+  }
+
+  /**
+   * Tests detecting a lying upgrade plugin cke4_button annotation.
+   */
+  public function testLyingUpgradePluginForCKEditor4Button(): void {
+    $this->enableModules(['ckeditor4to5upgrade_plugin_test']);
+    \Drupal::state()->set('ckeditor4to5upgrade_plugin_test', 'lying_button');
+
+    $this->expectException(\LogicException::class);
+    $this->expectExceptionMessage('The "foo" CKEditor4To5Upgrade plugin claims to provide an upgrade path for the "foo" CKEditor 4 button but does not.');
+
+    $this->upgradePluginManager->mapCKEditor4ToolbarButtonToCKEditor5ToolbarItem('foo');
+  }
+
+  /**
+   * Tests that only one plugin can provide an upgrade path for plugin settings.
+   */
+  public function testOnlyOneUpgradePluginAllowedPerCKEditor4PluginSettings(): void {
+    $this->enableModules(['ckeditor4to5upgrade_plugin_test']);
+    \Drupal::state()->set('ckeditor4to5upgrade_plugin_test', 'duplicate_plugin_settings');
+
+    $this->expectException(\OutOfBoundsException::class);
+    $this->expectExceptionMessage('The "stylescombo" CKEditor 4 plugin\'s settings are already being upgraded by the "core" CKEditor4To5Upgrade plugin, the "foo" plugin is as well. This conflict needs to be resolved.');
+
+    $this->upgradePluginManager->mapCKEditor4ToolbarButtonToCKEditor5ToolbarItem('foo');
+  }
+
+  /**
+   * Tests detecting a lying upgrade plugin cke4_plugin_settings annotation.
+   */
+  public function testLyingUpgradePluginForCKEditor4PluginSettings(): void {
+    $this->enableModules(['ckeditor4to5upgrade_plugin_test']);
+    \Drupal::state()->set('ckeditor4to5upgrade_plugin_test', 'lying_plugin_settings');
+
+    $this->expectException(\LogicException::class);
+    $this->expectExceptionMessage('The "foo" CKEditor4To5Upgrade plugin claims to provide an upgrade path for the "foo" CKEditor 4 plugin settings but does not.');
+
+    $this->upgradePluginManager->mapCKEditor4SettingsToCKEditor5Configuration('foo', []);
+  }
+
+}
diff --git a/core/modules/ckeditor5/tests/src/Kernel/CKEditor5PluginManagerTest.php b/core/modules/ckeditor5/tests/src/Kernel/CKEditor5PluginManagerTest.php
index 88ffa0191c..0148a45183 100644
--- a/core/modules/ckeditor5/tests/src/Kernel/CKEditor5PluginManagerTest.php
+++ b/core/modules/ckeditor5/tests/src/Kernel/CKEditor5PluginManagerTest.php
@@ -221,20 +221,46 @@ public function providerTestInvalidPluginDefinitions(): \Generator {
     label: "Foo bar"
     elements: {}
 YAML,
-      'The "ckeditor5_invalid_plugin_foo_bar" CKEditor 5 plugin definition has a "drupal.elements" key that is neither a list of HTML tags/attributes nor false.',
+      'The "ckeditor5_invalid_plugin_foo_bar" CKEditor 5 plugin definition has a "drupal.elements" value that is neither a list of HTML tags/attributes nor false.',
     ];
 
-    yield 'fixed drupal.elements' => [
+    yield 'wrongly fixed drupal.elements: no valid tags' => [
       <<<YAML
 ckeditor5_invalid_plugin_foo_bar:
   ckeditor5:
     plugins: {}
   drupal:
     label: "Foo bar"
-    # @todo Validate the shape of each string as well https://www.drupal.org/project/ckeditor5/issues/3231327.
     elements:
       - foo
       - bar
+YAML,
+      'The "ckeditor5_invalid_plugin_foo_bar" CKEditor 5 plugin definition has a value at "drupal.elements.0" that is not an HTML tag with optional attributes: "foo". Expected structure: "<tag allowedAttribute="allowedValue1 allowedValue2">".',
+    ];
+
+    yield 'wrongly fixed drupal.elements: multiple tags per entry' => [
+      <<<YAML
+ckeditor5_invalid_plugin_foo_bar:
+  ckeditor5:
+    plugins: {}
+  drupal:
+    label: "Foo bar"
+    elements:
+      - <foo> <bar>
+YAML,
+      'The "ckeditor5_invalid_plugin_foo_bar" CKEditor 5 plugin definition has a value at "drupal.elements.0": multiple tags listed, should be one: "<foo> <bar>".',
+    ];
+
+    yield 'fixed drupal.elements' => [
+      <<<YAML
+ckeditor5_invalid_plugin_foo_bar:
+  ckeditor5:
+    plugins: {}
+  drupal:
+    label: "Foo bar"
+    elements:
+      - <foo>
+      - <bar>
 YAML,
       NULL,
     ];
@@ -259,8 +285,8 @@ public function providerTestInvalidPluginDefinitions(): \Generator {
   drupal:
     label: "Foo bar"
     elements:
-      - foo
-      - bar
+      - <foo>
+      - <bar>
     admin_library: ckeditor5/foo_bar
 YAML,
       'The "ckeditor5_invalid_plugin_foo_bar" CKEditor 5 plugin definition has a "drupal.admin_library" key whose asset library "ckeditor5/foo_bar" does not exist.',
@@ -274,8 +300,8 @@ public function providerTestInvalidPluginDefinitions(): \Generator {
   drupal:
     label: "Foo bar"
     elements:
-      - foo
-      - bar
+      - <foo>
+      - <bar>
     admin_library: ckeditor5/admin.basic
 YAML,
       NULL,
@@ -330,12 +356,14 @@ public function testEnabledPlugins() {
       'ckeditor5_essentials',
       'ckeditor5_heading',
       'ckeditor5_paragraph',
+      'ckeditor5_pasteFromOffice',
     ];
     $this->assertSame($default_plugins, $definitions, 'No CKEditor 5 plugins found besides the built-in ones.');
     $default_libraries = [
       'ckeditor5/drupal.ckeditor5',
       'core/ckeditor5.basic',
       'core/ckeditor5.internal',
+      'core/ckeditor5.pasteFromOffice',
       'core/drupal.ckeditor5.emphasis',
     ];
     $this->assertSame($default_libraries, $this->manager->getEnabledLibraries($editor));
@@ -396,6 +424,7 @@ public function testEnabledPlugins() {
       'ckeditor5_heading',
       'ckeditor5_htmlSupport',
       'ckeditor5_paragraph',
+      'ckeditor5_pasteFromOffice',
     ];
     $this->assertSame($default_plugins, $definitions, 'No CKEditor 5 plugins found besides the built-in ones.');
     $default_libraries = [
@@ -403,6 +432,7 @@ public function testEnabledPlugins() {
       'core/ckeditor5.basic',
       'core/ckeditor5.htmlSupport',
       'core/ckeditor5.internal',
+      'core/ckeditor5.pasteFromOffice',
       'core/drupal.ckeditor5.emphasis',
     ];
     $this->assertSame($default_libraries, $this->manager->getEnabledLibraries($editor));
@@ -450,7 +480,7 @@ public function testProvidedElements(array $plugins, array $text_editor_settings
   /**
    * Provides uses cases enabling different elements and the expected results.
    */
-  public function providerTestProvidedElements() : array {
+  public function providerTestProvidedElements(): array {
     $text_align_classes = [
       'text-align-left' => TRUE,
       'text-align-center' => TRUE,
diff --git a/core/modules/ckeditor5/tests/src/Kernel/CKEditor5ValidationTestTrait.php b/core/modules/ckeditor5/tests/src/Kernel/CKEditor5ValidationTestTrait.php
index bdb4bcfa28..16c43e5ba0 100644
--- a/core/modules/ckeditor5/tests/src/Kernel/CKEditor5ValidationTestTrait.php
+++ b/core/modules/ckeditor5/tests/src/Kernel/CKEditor5ValidationTestTrait.php
@@ -28,7 +28,7 @@ trait CKEditor5ValidationTestTrait {
    *
    * @see \Drupal\ckeditor5\Plugin\Editor\CKEditor5::validatePair
    */
-  private function validatePairToViolationsArray(EditorInterface $text_editor, FilterFormatInterface $text_format, bool $all_compatibility_problems) : array {
+  private function validatePairToViolationsArray(EditorInterface $text_editor, FilterFormatInterface $text_format, bool $all_compatibility_problems): array {
     $violations = CKEditor5::validatePair($text_editor, $text_format, $all_compatibility_problems);
     $actual_violations = [];
     foreach ($violations as $violation) {
diff --git a/core/modules/ckeditor5/tests/src/Kernel/ConfigurablePluginTest.php b/core/modules/ckeditor5/tests/src/Kernel/ConfigurablePluginTest.php
index 3e0563340e..eb055f2900 100644
--- a/core/modules/ckeditor5/tests/src/Kernel/ConfigurablePluginTest.php
+++ b/core/modules/ckeditor5/tests/src/Kernel/ConfigurablePluginTest.php
@@ -39,7 +39,7 @@ protected function setUp(): void {
    */
   public function testDefaults() {
     $all_definitions = $this->manager->getDefinitions();
-    $configurable_definitions = array_filter($all_definitions, function (CKEditor5PluginDefinition $definition) : bool {
+    $configurable_definitions = array_filter($all_definitions, function (CKEditor5PluginDefinition $definition): bool {
       return $definition->isConfigurable();
     });
     $default_plugin_settings = [];
diff --git a/core/modules/ckeditor5/tests/src/Kernel/SmartDefaultSettingsTest.php b/core/modules/ckeditor5/tests/src/Kernel/SmartDefaultSettingsTest.php
index 4d719a5475..72442d424b 100644
--- a/core/modules/ckeditor5/tests/src/Kernel/SmartDefaultSettingsTest.php
+++ b/core/modules/ckeditor5/tests/src/Kernel/SmartDefaultSettingsTest.php
@@ -47,6 +47,7 @@ class SmartDefaultSettingsTest extends KernelTestBase {
    */
   protected static $modules = [
     'ckeditor',
+    'ckeditor_test',
     'ckeditor5',
     'editor',
     'filter',
@@ -93,6 +94,71 @@ protected function setUp(): void {
         ],
       ],
     ])->save();
+
+    FilterFormat::create([
+      'format' => 'cke4_plugins_with_settings',
+      'name' => 'All CKEditor 4 core plugins with settings',
+    ])->save();
+    Editor::create([
+      'format' => 'cke4_plugins_with_settings',
+      'editor' => 'ckeditor',
+      'settings' => [
+        'toolbar' => [
+          'rows' => [
+            0 => [
+              [
+                'name' => 'Buttons with settings',
+                'items' => [
+                  'Language',
+                ],
+              ],
+              [
+                'name' => 'Button without upgrade path',
+                'items' => [
+                  'Llama',
+                ],
+              ],
+            ],
+          ],
+        ],
+        'plugins' => [
+          'language' => [
+            'language_list' => 'all',
+          ],
+          // Plugin setting without upgrade path.
+          'llama_contextual_and_button' => [
+            'ultra_llama_mode' => TRUE,
+          ],
+        ],
+      ],
+    ])->save();
+
+    FilterFormat::create([
+      'format' => 'cke4_plugins_with_settings_for_disabled_plugins',
+      'name' => 'All CKEditor 4 core plugins with settings for disabled plugins',
+    ])->save();
+    Editor::create([
+      'format' => 'cke4_plugins_with_settings_for_disabled_plugins',
+      'editor' => 'ckeditor',
+      'settings' => [
+        // Empty toolbar.
+        'toolbar' => [
+          'rows' => [
+            0 => [
+              [
+                'name' => 'Only buttons without settings',
+                'items' => [
+                  'Bold',
+                ],
+              ],
+            ],
+          ],
+        ],
+        // Same plugin settings as `cke4_plugins_with_settings`.
+        'plugins' => Editor::load('cke4_plugins_with_settings')->getSettings()['plugins'],
+      ],
+    ])->save();
+
   }
 
   /**
@@ -112,16 +178,15 @@ protected function setUp(): void {
    * @param array $expected_fundamental_compatibility_violations
    *   All expected fundamental compatibility violations for the given text
    *   format.
+   * @param string[] $expected_messages
+   *   The expected messages associated with the computed settings.
    * @param array|null $expected_post_filter_drop_fundamental_compatibility_violations
    *   All expected fundamental compatibility violations for the given text
    *   format, after dropping filters specified in $filters_to_drop.
    *
    * @dataProvider provider
-   *
-   * @todo In https://www.drupal.org/project/ckeditor5/issues/3226694 this
-   *   should also test the expected messages.
    */
-  public function test(string $format_id, array $filters_to_drop, array $expected_ckeditor5_settings, string $expected_superset, array $expected_fundamental_compatibility_violations, ?array $expected_post_filter_drop_fundamental_compatibility_violations = NULL) : void {
+  public function test(string $format_id, array $filters_to_drop, array $expected_ckeditor5_settings, string $expected_superset, array $expected_fundamental_compatibility_violations, array $expected_messages, ?array $expected_post_filter_drop_fundamental_compatibility_violations = NULL): void {
     $text_format = FilterFormat::load($format_id);
     $text_editor = Editor::load($format_id);
 
@@ -155,7 +220,7 @@ public function test(string $format_id, array $filters_to_drop, array $expected_
       }
     }
 
-    $updated_text_editor = $this->smartDefaultSettings->computeSmartDefaultSettings($text_editor, $text_format);
+    [$updated_text_editor, $messages] = $this->smartDefaultSettings->computeSmartDefaultSettings($text_editor, $text_format);
 
     // Ensure that the result of ::computeSmartDefaultSettings() always complies
     // with the config schema.
@@ -211,6 +276,11 @@ public function test(string $format_id, array $filters_to_drop, array $expected_
 
     // The resulting pair should be valid.
     $this->assertSame([], $this->validatePairToViolationsArray($updated_text_editor, $updated_text_format, TRUE));
+
+    foreach ($messages as $key => $message) {
+      $messages[$key] = (string) $message;
+    }
+    $this->assertSame($expected_messages, $messages);
   }
 
   /**
@@ -283,11 +353,22 @@ public function provider() {
       ],
       'expected_superset' => '<span lang dir>',
       'expected_fundamental_compatibility_violations' => [],
+      'expected_messages' => [
+        'The following plugins were enabled to support tags that are allowed by this text format: <em class="placeholder">Code (for tags: &lt;code&gt;) Language (for tags: &lt;span&gt;)</em>.',
+        'The following tags were permitted by this format\'s filter configuration, but no plugin was available that supports them. To ensure the tags remain supported by this text format, the following were added to the Source Editing plugin\'s <em>Manually editable HTML tags</em>: &lt;cite&gt; &lt;dl&gt; &lt;dt&gt; &lt;dd&gt;.',
+      ],
     ];
 
-    yield "basic_html can be switched to CKEditor 5 without problems" => $basic_html_test_case;
+    yield "basic_html can be switched to CKEditor 5 without problems (3 upgrade messages)" => NestedArray::mergeDeep(
+      $basic_html_test_case,
+      [
+        'expected_messages' => [
+          'This format\'s HTML filters includes plugins that support the following tags, but not some of their attributes. To ensure these attributes remain supported by this text format, the following were added to the Source Editing plugin\'s <em>Manually editable HTML tags</em>: &lt;a hreflang&gt; &lt;blockquote cite&gt; &lt;ul type&gt; &lt;ol start type&gt; &lt;h2 id&gt; &lt;h3 id&gt; &lt;h4 id&gt; &lt;h5 id&gt; &lt;h6 id&gt;.',
+        ],
+      ]
+    );
 
-    yield "basic_html with filter_caption removed => disallows <img data-caption>" => NestedArray::mergeDeep(
+    yield "basic_html with filter_caption removed => disallows <img data-caption> => supported through sourceEditing (3 upgrade messages)" => NestedArray::mergeDeep(
       $basic_html_test_case,
       [
         'filters_to_drop' => [
@@ -302,9 +383,12 @@ public function provider() {
             ],
           ],
         ],
+        'expected_messages' => [
+          'This format\'s HTML filters includes plugins that support the following tags, but not some of their attributes. To ensure these attributes remain supported by this text format, the following were added to the Source Editing plugin\'s <em>Manually editable HTML tags</em>: &lt;a hreflang&gt; &lt;blockquote cite&gt; &lt;ul type&gt; &lt;ol start type&gt; &lt;h2 id&gt; &lt;h3 id&gt; &lt;h4 id&gt; &lt;h5 id&gt; &lt;h6 id&gt; &lt;img data-caption&gt;.',
+        ],
       ]);
 
-    yield "basic_html with filter_align removed => disallows <img data-align>, but " => NestedArray::mergeDeep(
+    yield "basic_html with filter_align removed => disallows <img data-align> => supported through sourceEditing (3 upgrade messages) " => NestedArray::mergeDeep(
       $basic_html_test_case,
       [
         'filters_to_drop' => [
@@ -319,9 +403,12 @@ public function provider() {
             ],
           ],
         ],
+        'expected_messages' => [
+          'This format\'s HTML filters includes plugins that support the following tags, but not some of their attributes. To ensure these attributes remain supported by this text format, the following were added to the Source Editing plugin\'s <em>Manually editable HTML tags</em>: &lt;a hreflang&gt; &lt;blockquote cite&gt; &lt;ul type&gt; &lt;ol start type&gt; &lt;h2 id&gt; &lt;h3 id&gt; &lt;h4 id&gt; &lt;h5 id&gt; &lt;h6 id&gt; &lt;img data-align&gt;.',
+        ],
       ]);
 
-    yield "restricted_html can be switched to CKEditor 5 after dropping the two markup-creating filters" => [
+    yield "restricted_html can be switched to CKEditor 5 after dropping the two markup-creating filters (3 upgrade messages)" => [
       'format_id' => 'restricted_html',
       'filters_to_drop' => [
         'filter_autop' => TRUE,
@@ -386,12 +473,17 @@ public function provider() {
           1 => 'CKEditor 5 only works with HTML-based text formats. The "<em class="placeholder">Convert line breaks into HTML (i.e. &lt;code&gt;&amp;lt;br&amp;gt;&lt;/code&gt; and &lt;code&gt;&amp;lt;p&amp;gt;&lt;/code&gt;)</em>" (<em class="placeholder">filter_autop</em>) filter implies this text format is not HTML anymore.',
         ],
       ],
+      'expected_messages' => [
+        'The following plugins were enabled to support tags that are allowed by this text format: <em class="placeholder">Link (for tags: &lt;a&gt;) Block quote (for tags: &lt;blockquote&gt;) Code (for tags: &lt;code&gt;) List (for tags: &lt;ul&gt;&lt;ol&gt;&lt;li&gt;)</em>.',
+        'The following tags were permitted by this format\'s filter configuration, but no plugin was available that supports them. To ensure the tags remain supported by this text format, the following were added to the Source Editing plugin\'s <em>Manually editable HTML tags</em>: &lt;cite&gt; &lt;dl&gt; &lt;dt&gt; &lt;dd&gt;.',
+        'This format\'s HTML filters includes plugins that support the following tags, but not some of their attributes. To ensure these attributes remain supported by this text format, the following were added to the Source Editing plugin\'s <em>Manually editable HTML tags</em>: &lt;a hreflang&gt; &lt;blockquote cite&gt; &lt;ul type&gt; &lt;ol start type&gt; &lt;h2 id&gt; &lt;h3 id&gt; &lt;h4 id&gt; &lt;h5 id&gt; &lt;h6 id&gt;.',
+      ],
       'expected_post_filter_drop_fundamental_compatibility_violations' => [
         '' => 'CKEditor 5 needs at least the &lt;p&gt; and &lt;br&gt; tags to be allowed to be able to function. They are not allowed by the "<em class="placeholder">Limit allowed HTML tags and correct faulty HTML</em>" (<em class="placeholder">filter_html</em>) filter.',
       ],
     ];
 
-    yield "full_html can be switched to CKEditor 5 after dropping the two markup-creating filters" => [
+    yield "full_html can be switched to CKEditor 5 (no upgrade messages)" => [
       'format_id' => 'full_html',
       'filters_to_drop' => [],
       'expected_ckeditor5_settings' => [
@@ -436,9 +528,10 @@ public function provider() {
       ],
       'expected_superset' => '',
       'expected_fundamental_compatibility_violations' => [],
+      'expected_messages' => [],
     ];
 
-    yield "filter_only__filter_html can be switched to CKEditor 5 without problems" => [
+    yield "filter_only__filter_html can be switched to CKEditor 5 without problems (3 upgrade messages)" => [
       'format_id' => 'filter_only__filter_html',
       'filters_to_drop' => [],
       'expected_ckeditor5_settings' => [
@@ -491,6 +584,50 @@ public function provider() {
       'expected_fundamental_compatibility_violations' => [
         '' => 'CKEditor 5 needs at least the &lt;p&gt; and &lt;br&gt; tags to be allowed to be able to function. They are not allowed by the "<em class="placeholder">Limit allowed HTML tags and correct faulty HTML</em>" (<em class="placeholder">filter_html</em>) filter.',
       ],
+      'expected_messages' => [
+        'The following plugins were enabled to support tags that are allowed by this text format: <em class="placeholder">Link (for tags: &lt;a&gt;) Block quote (for tags: &lt;blockquote&gt;) Code (for tags: &lt;code&gt;) List (for tags: &lt;ul&gt;&lt;ol&gt;&lt;li&gt;)</em>.',
+        'The following tags were permitted by this format\'s filter configuration, but no plugin was available that supports them. To ensure the tags remain supported by this text format, the following were added to the Source Editing plugin\'s <em>Manually editable HTML tags</em>: &lt;cite&gt; &lt;dl&gt; &lt;dt&gt; &lt;dd&gt;.',
+        'This format\'s HTML filters includes plugins that support the following tags, but not some of their attributes. To ensure these attributes remain supported by this text format, the following were added to the Source Editing plugin\'s <em>Manually editable HTML tags</em>: &lt;a hreflang&gt; &lt;blockquote cite&gt; &lt;ul type&gt; &lt;ol start type=&quot;1 A I&quot;&gt; &lt;h2 id=&quot;jump-*&quot;&gt; &lt;h3 id&gt; &lt;h4 id&gt; &lt;h5 id&gt; &lt;h6 id&gt;.',
+      ],
+    ];
+
+    yield "cke4_plugins_with_settings can be switched to CKEditor 5 without problems, settings are upgraded too" => [
+      'format_id' => 'cke4_plugins_with_settings',
+      'filters_to_drop' => [],
+      'expected_ckeditor5_settings' => [
+        'toolbar' => [
+          'items' => [
+            'textPartLanguage',
+          ],
+        ],
+        'plugins' => [
+          'ckeditor5_language' => [
+            'language_list' => 'all',
+          ],
+        ],
+      ],
+      'expected_superset' => '',
+      'expected_fundamental_compatibility_violations' => [],
+      'expected_messages' => [
+        'The CKEditor 4 button <em class="placeholder">Llama</em> does not have a known upgrade path. If it allowed editing markup, then you can do so now through the Source Editing functionality.',
+        'The <em class="placeholder">llama_contextual_and_button</em> plugin settings do not have a known upgrade path.',
+      ],
+    ];
+
+    yield "cke4_plugins_with_settings_for_disabled_plugins can be switched to CKEditor 5 without problems; irrelevant settings are dropped" => [
+      'format_id' => 'cke4_plugins_with_settings_for_disabled_plugins',
+      'filters_to_drop' => [],
+      'expected_ckeditor5_settings' => [
+        'toolbar' => [
+          'items' => [
+            'bold',
+          ],
+        ],
+        'plugins' => [],
+      ],
+      'expected_superset' => '',
+      'expected_fundamental_compatibility_violations' => [],
+      'expected_messages' => [],
     ];
   }
 
diff --git a/core/modules/ckeditor5/tests/src/Kernel/ValidatorsTest.php b/core/modules/ckeditor5/tests/src/Kernel/ValidatorsTest.php
index 3ea74ed6f4..88d84e05ed 100644
--- a/core/modules/ckeditor5/tests/src/Kernel/ValidatorsTest.php
+++ b/core/modules/ckeditor5/tests/src/Kernel/ValidatorsTest.php
@@ -11,6 +11,7 @@
 use Drupal\filter\FilterFormatInterface;
 use Drupal\KernelTests\KernelTestBase;
 use Drupal\Tests\SchemaCheckTestTrait;
+use Symfony\Component\Yaml\Yaml;
 
 /**
  * @covers \Drupal\ckeditor5\Plugin\Validation\Constraint\ToolbarItemConstraintValidator
@@ -104,7 +105,7 @@ public function test(array $ckeditor5_settings, array $expected_violations) {
   /**
    * Provides a list of Text Editor config entities using CKEditor 5 to test.
    */
-  public function provider() : array {
+  public function provider(): array {
     $data = [];
     $data['CKEditor5::getDefaultSettings()'] = [
       // @see ::test()
@@ -307,7 +308,7 @@ public function testPair(array $ckeditor5_settings, array $editor_image_upload_s
   /**
    * Provides a list of Text Editor + Text Format pairs to test.
    */
-  public function providerPair() : array {
+  public function providerPair(): array {
     // cspell:ignore donk
     $data = [];
     $data['INVALID: non-HTML format: filter_autop'] = [
@@ -399,39 +400,7 @@ public function providerPair() : array {
         '' => 'CKEditor 5 needs at least the &lt;p&gt; and &lt;br&gt; tags to be allowed to be able to function. They are forbidden by the "<em class="placeholder">Tag and attribute restricting filter</em>" (<em class="placeholder">filter_test_restrict_tags_and_attributes</em>) filter.',
       ],
     ];
-    // @todo read this directly from
-    // core/profiles/standard/config/install/filter.format.restricted_html.yml,
-    // but it seems that PHPUnit forbids us from doing that
-    // https://www.drupal.org/project/ckeditor5/issues/3231325.
-    $restricted_html_format_filters = [
-      'filter_html' => [
-        'id' => 'filter_html',
-        'provider' => 'filter',
-        'status' => TRUE,
-        'weight' => -10,
-        'settings' => [
-          'allowed_html' => "<a href hreflang> <em> <strong> <cite> <blockquote cite> <code> <ul type> <ol start type> <li> <dl> <dt> <dd> <h2 id> <h3 id> <h4 id> <h5 id> <h6 id>",
-          'filter_html_help' => TRUE,
-          'filter_html_nofollow' => TRUE,
-        ],
-      ],
-      'filter_autop' => [
-        'id' => 'filter_autop',
-        'provider' => 'filter',
-        'status' => TRUE,
-        'weight' => 0,
-        'settings' => [],
-      ],
-      'filter_url' => [
-        'id' => 'filter_url',
-        'provider' => 'filter',
-        'status' => TRUE,
-        'weight' => 0,
-        'settings' => [
-          'filter_url_length' => 72,
-        ],
-      ],
-    ];
+    $restricted_html_format_filters = Yaml::parseFile('profiles/standard/config/install/filter.format.restricted_html.yml')['filters'];
     $data['INVALID: the default restricted_html text format'] = [
       'settings' => [
         'toolbar' => [
diff --git a/core/webpack.config.js b/core/webpack.config.js
index 37d4649bd3..bca8e63674 100644
--- a/core/webpack.config.js
+++ b/core/webpack.config.js
@@ -11,9 +11,9 @@ function getDirectories(srcpath) {
 }
 
 module.exports = [];
-// Loop through every subdirectory in src, which should be a different
+// Loop through every subdirectory in ckeditor5_plugins, which should be a different
 // plugin, and build them all in ./build.
-getDirectories('assets/vendor/ckeditor5/drupal/src').forEach((dir) => {
+getDirectories('assets/vendor/ckeditor5/ckeditor5_plugins').forEach((dir) => {
   const bc = {
     mode: 'production',
     optimization: {
@@ -32,10 +32,10 @@ getDirectories('assets/vendor/ckeditor5/drupal/src').forEach((dir) => {
       moduleIds: 'named',
     },
     entry: {
-      path: path.resolve(__dirname, 'assets/vendor/ckeditor5/drupal/src', dir, 'src/index.js'),
+      path: path.resolve(__dirname, 'assets/vendor/ckeditor5/ckeditor5_plugins', dir, 'src/index.js'),
     },
     output: {
-      path: path.resolve(__dirname, 'assets/vendor/ckeditor5/build/drupal'),
+      path: path.resolve(__dirname, 'assets/vendor/ckeditor5/build/ckeditor5_plugins'),
       filename: `${dir}.js`,
       library: ['CKEditor5', dir],
       libraryTarget: 'umd',