diff --git a/core/modules/ckeditor5/ckeditor5.libraries.yml b/core/modules/ckeditor5/ckeditor5.libraries.yml
index f458a46ea4..d0908635c0 100644
--- a/core/modules/ckeditor5/ckeditor5.libraries.yml
+++ b/core/modules/ckeditor5/ckeditor5.libraries.yml
@@ -29,6 +29,8 @@ drupal.ckeditor5:
     - editor/drupal.editor
     - ckeditor5/drupal.ckeditor5.quickedit-temporary-work-around
     - ckeditor5/drupal.ckeditor5.stylesheets
+    - core/drupalSettings
+    - core/drupal.message
 
 # Library used for dynamically loading CKEditor 5 stylesheets from the default
 # front end theme.
@@ -95,6 +97,8 @@ drupal.ckeditor5.filter.admin:
     - core/drupal.message
     - core/once
     - core/drupal.ajax
+    - core/drupalSettings
+    - core/drupal.message
 
 admin:
   js:
diff --git a/core/modules/ckeditor5/ckeditor5.module b/core/modules/ckeditor5/ckeditor5.module
index fa4ec4c2ce..c57091491b 100644
--- a/core/modules/ckeditor5/ckeditor5.module
+++ b/core/modules/ckeditor5/ckeditor5.module
@@ -355,6 +355,38 @@ function ckeditor5_library_info_alter(&$libraries, $extension) {
         }, array_flip($css)),
       ],
     ];
+
+    $admin_theme = \Drupal::configFactory()->get('system.theme')->get('admin');
+    $admin_theme_info = \Drupal::service('theme_handler')->listInfo()[$admin_theme]->info;
+
+    $opt_out_ckeditor5_stylesheets = (isset($admin_theme_info['ckeditor5-stylesheets']) && $admin_theme_info['ckeditor5-info'] !== FALSE);
+
+    // Check if:
+    // - We are on an admin page.
+    // - No CKEditor 5 stylesheets were discovered.
+    // - The admin theme has not explicitly opted out of CKEditor 5 stylesheets
+    //   by setting its ckeditor5-stylesheets property to FALSE.
+    // If the following are all true, we check if any ckeditor_stylesheets are
+    // being added to the default theme. If so, then a warning is generated to
+    // make users aware that stylesheets they expect to load with their editor
+    // may not be present.
+    if (\Drupal::service('router.admin_context')->isAdminRoute() && empty($css) && !$opt_out_ckeditor5_stylesheets) {
+      $default_theme = \Drupal::configFactory()->get('system.theme')->get('default');
+      $default_theme_info = \Drupal::service('theme_handler')->listInfo()[$default_theme]->info;
+      $ckeditor4_stylesheets = $default_theme_info['ckeditor_stylesheets'] ?? [];
+
+      $base_theme = $default_theme_info['base_theme'] ?? FALSE;
+      while ($base_theme) {
+        $base_theme_info = \Drupal::service('theme_handler')->listInfo()[$base_theme]->info;
+        $base_theme_ckeditor4_stylesheets = $base_theme_info['ckeditor_stylesheets'] ?? [];
+        $ckeditor4_stylesheets = array_merge($ckeditor4_stylesheets, $base_theme_ckeditor4_stylesheets);
+        $base_theme = $base_theme_info['base_theme'] ?? FALSE;
+      }
+      if (!empty($ckeditor4_stylesheets)) {
+        $libraries['drupal.ckeditor5']['drupalSettings']['ckeditor5']['ckeditor4_stylesheets'] = $ckeditor4_stylesheets;
+        $libraries['drupal.ckeditor5.filter.admin']['drupalSettings']['ckeditor5']['ckeditor4_stylesheets'] = $ckeditor4_stylesheets;
+      }
+    }
   }
 
   if ($extension === 'core') {
diff --git a/core/modules/ckeditor5/js/ckeditor5.es6.js b/core/modules/ckeditor5/js/ckeditor5.es6.js
index 6716e8d2c0..a21b543a79 100644
--- a/core/modules/ckeditor5/js/ckeditor5.es6.js
+++ b/core/modules/ckeditor5/js/ckeditor5.es6.js
@@ -3,7 +3,7 @@
  * CKEditor 5 implementation of {@link Drupal.editors} API.
  */
 /* global CKEditor5 */
-((Drupal, debounce, CKEditor5, $) => {
+((Drupal, debounce, CKEditor5, $, drupalSettings) => {
   /**
    * The CKEDITOR instances.
    *
@@ -354,6 +354,23 @@
           // eslint-disable-next-line no-console
           console.error(error);
         });
+
+      if (
+        drupalSettings.ckeditor5 &&
+        drupalSettings.ckeditor5.ckeditor4_stylesheets
+      ) {
+        const editorMessageContainer = document.createElement('div');
+        element.parentNode.insertBefore(editorMessageContainer, element);
+        const editorMessages = new Drupal.Message(editorMessageContainer);
+        editorMessages.add(
+          Drupal.t(
+            'There are CKEditor 4 stylesheets that may need CKeditor 5 equivalents.',
+          ),
+          {
+            type: 'warning',
+          },
+        );
+      }
     },
 
     /**
@@ -580,4 +597,4 @@
       Drupal.ckeditor5.saveCallback = null;
     }
   });
-})(Drupal, Drupal.debounce, CKEditor5, jQuery);
+})(Drupal, Drupal.debounce, CKEditor5, jQuery, drupalSettings);
diff --git a/core/modules/ckeditor5/js/ckeditor5.filter.admin.es6.js b/core/modules/ckeditor5/js/ckeditor5.filter.admin.es6.js
index 69b8643905..7735acb179 100644
--- a/core/modules/ckeditor5/js/ckeditor5.filter.admin.es6.js
+++ b/core/modules/ckeditor5/js/ckeditor5.filter.admin.es6.js
@@ -3,7 +3,7 @@
  * Provides Text Editor UI improvements specific to CKEditor 5.
  */
 
-((Drupal, once) => {
+((Drupal, once, drupalSettings) => {
   Drupal.behaviors.allowedTagsListener = {
     attach: function attach(context) {
       once(
@@ -111,6 +111,113 @@
     },
   };
 
+  Drupal.behaviors.ckEditor5StylesheetsWarn = {
+    attach: function attach() {
+      const editorSelect = once(
+        'editor-select-stylesheet-warning',
+        document.querySelector(
+          '#filter-format-edit-form #edit-editor-editor, #filter-format-add-form #edit-editor-editor',
+        ),
+      );
+
+      if (
+        typeof editorSelect[0] !== 'undefined' &&
+        drupalSettings.ckeditor5 &&
+        drupalSettings.ckeditor5.ckeditor4_stylesheets
+      ) {
+        const select = editorSelect[0];
+
+        // Add a container for messages above the text format select element.
+        const selectMessageContainer = document.createElement('div');
+        select.parentNode.insertBefore(selectMessageContainer, select);
+        const selectMessages = new Drupal.Message(selectMessageContainer);
+        const editorSettings = document.querySelector(
+          '#editor-settings-wrapper',
+        );
+
+        /**
+         * Adds a ckeditor_stylesheets warning to the message container.
+         */
+        const ck5Warning = () => {
+          selectMessages.add(
+            Drupal.t(
+              'There are CKEditor 4 stylesheets without equivalents: @stylesheets',
+              {
+                '@stylesheets':
+                  drupalSettings.ckeditor5.ckeditor4_stylesheets.join(', '),
+              },
+            ),
+            {
+              type: 'warning',
+            },
+          );
+        };
+
+        /**
+         * Adds a warning if the selected editor is ckeditor5, otherwise clears
+         * the message container.
+         */
+        const updateWarningStatus = () => {
+          if (
+            select.value === 'ckeditor5' &&
+            !select.classList.contains('error')
+          ) {
+            ck5Warning();
+          } else {
+            editorSettings.hidden = false;
+            selectMessages.clear();
+          }
+        };
+
+        const selectChangeHandler = () => {
+          // Declare the observer first so the observer callback can access it.
+          let editorSelectObserver = null;
+
+          /**
+           * MutationObserver callback for the editor select.
+           *
+           * This listens for the removal 'disabled' attribute on the <select>,
+           * which means the AJAX callback has completed and the form is in a
+           * state suitable for seeing if the IE11 warning is needed.
+           *
+           * @param {Array} mutations
+           *   The element's mutations.
+           */
+          function whenSelectAttributeChanges(mutations) {
+            for (let i = 0; i < mutations.length; i++) {
+              // When the select input is no longer disabled, the AJAX request
+              // is complete and the UI is in a state where it can be determined
+              // if the IE11 warning is needed.
+              if (
+                mutations[i].type === 'attributes' &&
+                mutations[i].attributeName === 'disabled' &&
+                !select.disabled
+              ) {
+                updateWarningStatus();
+                editorSelectObserver.disconnect();
+              }
+            }
+          }
+
+          // An observer is used because during the select change event, it is
+          // not yet known if validation prevented the switch to CKEditor 5.
+          editorSelectObserver = new MutationObserver(
+            whenSelectAttributeChanges,
+          );
+          editorSelectObserver.observe(select, {
+            attributes: true,
+            attributeOldValue: true,
+          });
+        };
+
+        updateWarningStatus();
+
+        // Listen to text format selection changes.
+        select.addEventListener('change', selectChangeHandler);
+      }
+    },
+  };
+
   // Copy the function that is about to be overridden so it can be invoked
   // inside the override.
   const originalAjaxEventResponse = Drupal.Ajax.prototype.eventResponse;
@@ -153,4 +260,4 @@
 
     originalAjaxEventResponse.apply(this, args);
   };
-})(Drupal, once);
+})(Drupal, once, drupalSettings);
diff --git a/core/modules/ckeditor5/js/ckeditor5.filter.admin.js b/core/modules/ckeditor5/js/ckeditor5.filter.admin.js
index 1c8d44fd91..a43750732d 100644
--- a/core/modules/ckeditor5/js/ckeditor5.filter.admin.js
+++ b/core/modules/ckeditor5/js/ckeditor5.filter.admin.js
@@ -5,7 +5,7 @@
 * @preserve
 **/
 
-(function (Drupal, once) {
+(function (Drupal, once, drupalSettings) {
   Drupal.behaviors.allowedTagsListener = {
     attach: function attach(context) {
       once('allowed-tags-listener', context.querySelector('[data-drupal-selector="edit-filters-filter-html-settings-allowed-html"]')).forEach(function (textarea) {
@@ -69,6 +69,58 @@
       });
     }
   };
+  Drupal.behaviors.ckEditor5StylesheetsWarn = {
+    attach: function attach() {
+      var editorSelect = once('editor-select-stylesheet-warning', document.querySelector('#filter-format-edit-form #edit-editor-editor, #filter-format-add-form #edit-editor-editor'));
+
+      if (typeof editorSelect[0] !== 'undefined' && drupalSettings.ckeditor5 && drupalSettings.ckeditor5.ckeditor4_stylesheets) {
+        var select = editorSelect[0];
+        var selectMessageContainer = document.createElement('div');
+        select.parentNode.insertBefore(selectMessageContainer, select);
+        var selectMessages = new Drupal.Message(selectMessageContainer);
+        var editorSettings = document.querySelector('#editor-settings-wrapper');
+
+        var ck5Warning = function ck5Warning() {
+          selectMessages.add(Drupal.t('There are CKEditor 4 stylesheets without equivalents: @stylesheets', {
+            '@stylesheets': drupalSettings.ckeditor5.ckeditor4_stylesheets.join(', ')
+          }), {
+            type: 'warning'
+          });
+        };
+
+        var updateWarningStatus = function updateWarningStatus() {
+          if (select.value === 'ckeditor5' && !select.classList.contains('error')) {
+            ck5Warning();
+          } else {
+            editorSettings.hidden = false;
+            selectMessages.clear();
+          }
+        };
+
+        var selectChangeHandler = function selectChangeHandler() {
+          var editorSelectObserver = null;
+
+          function whenSelectAttributeChanges(mutations) {
+            for (var i = 0; i < mutations.length; i++) {
+              if (mutations[i].type === 'attributes' && mutations[i].attributeName === 'disabled' && !select.disabled) {
+                updateWarningStatus();
+                editorSelectObserver.disconnect();
+              }
+            }
+          }
+
+          editorSelectObserver = new MutationObserver(whenSelectAttributeChanges);
+          editorSelectObserver.observe(select, {
+            attributes: true,
+            attributeOldValue: true
+          });
+        };
+
+        updateWarningStatus();
+        select.addEventListener('change', selectChangeHandler);
+      }
+    }
+  };
   var originalAjaxEventResponse = Drupal.Ajax.prototype.eventResponse;
 
   Drupal.Ajax.prototype.eventResponse = function ckeditor5AjaxEventResponse() {
@@ -84,4 +136,4 @@
 
     originalAjaxEventResponse.apply(this, args);
   };
-})(Drupal, once);
\ No newline at end of file
+})(Drupal, once, drupalSettings);
\ No newline at end of file
diff --git a/core/modules/ckeditor5/js/ckeditor5.js b/core/modules/ckeditor5/js/ckeditor5.js
index 6e7a876e7a..2caac09606 100644
--- a/core/modules/ckeditor5/js/ckeditor5.js
+++ b/core/modules/ckeditor5/js/ckeditor5.js
@@ -33,7 +33,7 @@ function _arrayWithoutHoles(arr) { if (Array.isArray(arr)) return _arrayLikeToAr
 
 function _arrayLikeToArray(arr, len) { if (len == null || len > arr.length) len = arr.length; for (var i = 0, arr2 = new Array(len); i < len; i++) { arr2[i] = arr[i]; } return arr2; }
 
-(function (Drupal, debounce, CKEditor5, $) {
+(function (Drupal, debounce, CKEditor5, $, drupalSettings) {
   Drupal.CKEditor5Instances = new Map();
   var callbacks = new Map();
   var required = new Set();
@@ -220,6 +220,15 @@ function _arrayLikeToArray(arr, len) { if (len == null || len > arr.length) len
       }).catch(function (error) {
         console.error(error);
       });
+
+      if (drupalSettings.ckeditor5 && drupalSettings.ckeditor5.ckeditor4_stylesheets) {
+        var editorMessageContainer = document.createElement('div');
+        element.parentNode.insertBefore(editorMessageContainer, element);
+        var editorMessages = new Drupal.Message(editorMessageContainer);
+        editorMessages.add(Drupal.t('There are CKEditor 4 stylesheets that may need CKeditor 5 equivalents.'), {
+          type: 'warning'
+        });
+      }
     },
     detach: function detach(element, format, trigger) {
       var id = getElementId(element);
@@ -362,4 +371,4 @@ function _arrayLikeToArray(arr, len) { if (len == null || len > arr.length) len
       Drupal.ckeditor5.saveCallback = null;
     }
   });
-})(Drupal, Drupal.debounce, CKEditor5, jQuery);
\ No newline at end of file
+})(Drupal, Drupal.debounce, CKEditor5, jQuery, drupalSettings);
\ No newline at end of file
diff --git a/core/modules/ckeditor5/tests/src/FunctionalJavascript/CKEditorStylesheetsWarningTest.php b/core/modules/ckeditor5/tests/src/FunctionalJavascript/CKEditorStylesheetsWarningTest.php
new file mode 100644
index 0000000000..c9ebbcd373
--- /dev/null
+++ b/core/modules/ckeditor5/tests/src/FunctionalJavascript/CKEditorStylesheetsWarningTest.php
@@ -0,0 +1,47 @@
+<?php
+
+namespace Drupal\Tests\ckeditor5\FunctionalJavascript;
+
+/**
+ * Tests warnings when ckeditor_stylesheets do not have CKEditor 5 equivalents.
+ *
+ * @group ckeditor5
+ * @internal
+ */
+class CKEditorStylesheetsWarningTest extends CKEditor5TestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+    $theme_installer = \Drupal::service('theme_installer');
+    $theme_installer->install(['test_ckeditor_stylesheets_without_5']);
+    $this->config('system.theme')->set('default', 'test_ckeditor_stylesheets_without_5')->save();
+    $theme_installer->install(['stark']);
+    $this->config('system.theme')->set('admin', 'stark')->save();
+  }
+
+  /**
+   * Test the ckeditor_stylesheets warning in the filter UI.
+   */
+  public function testWarningFilterUI() {
+    $page = $this->getSession()->getPage();
+    $assert_session = $this->assertSession();
+    $this->addNewTextFormat($page, $assert_session);
+    $this->drupalGet('admin/config/content/formats/manage/ckeditor5');
+    $assert_session->waitForText('There are CKEditor 4 stylesheets without equivalents');
+  }
+
+  /**
+   * Test the ckeditor_stylesheets warning accompanying an editor instance.
+   */
+  public function testWarningEditorInstance() {
+    $page = $this->getSession()->getPage();
+    $assert_session = $this->assertSession();
+    $this->addNewTextFormat($page, $assert_session);
+    $this->drupalGet('node/add');
+    $assert_session->waitForText('There are CKEditor 4 stylesheets that may need CKeditor 5 equivalents.');
+  }
+
+}
diff --git a/core/modules/system/tests/themes/test_ckeditor_stylesheets_without_5/css/yokotsoko.css b/core/modules/system/tests/themes/test_ckeditor_stylesheets_without_5/css/yokotsoko.css
new file mode 100644
index 0000000000..3df54f5c99
--- /dev/null
+++ b/core/modules/system/tests/themes/test_ckeditor_stylesheets_without_5/css/yokotsoko.css
@@ -0,0 +1,4 @@
+/**
+ * @file
+ * Test CSS asset file for test_ckeditor_stylesheets_drupal_root.theme.
+ */
diff --git a/core/modules/system/tests/themes/test_ckeditor_stylesheets_without_5/test_ckeditor_stylesheets_without_5.info.yml b/core/modules/system/tests/themes/test_ckeditor_stylesheets_without_5/test_ckeditor_stylesheets_without_5.info.yml
new file mode 100644
index 0000000000..dbf31d37f5
--- /dev/null
+++ b/core/modules/system/tests/themes/test_ckeditor_stylesheets_without_5/test_ckeditor_stylesheets_without_5.info.yml
@@ -0,0 +1,9 @@
+name: Test CKEditor stylesheets without a setting for CKEditor 5
+type: theme
+description: 'A theme that uses ckeditor_stylesheets without ckeditor5-stylesheets.'
+package: Testing
+version: VERSION
+base theme: false
+
+ckeditor_stylesheets:
+  - /core/modules/system/tests/themes/test_ckeditor_stylesheets_without_5/css/yokotsoko.css