reverted:
--- b/core/misc/ajax.js
+++ a/core/misc/ajax.js
@@ -1326,18 +1326,16 @@
      *   The XMLHttpRequest status.
      */
     add_css: function (ajax, response, status) {
-      // Allow the response to set the document context.
-      var context = response.document || window.document;
       // Add the styles in the normal way.
+      $('head').prepend(response.data);
-      $(context).find('head').prepend(response.data);
       // Add imports in the styles using the addImport method if available.
       var match;
       var importMatch = /^@import url\("(.*)"\);$/igm;
+      if (document.styleSheets[0].addImport && importMatch.test(response.data)) {
-      if (context.styleSheets[0].addImport && importMatch.test(response.data)) {
         importMatch.lastIndex = 0;
         do {
           match = importMatch.exec(response.data);
+          document.styleSheets[0].addImport(match[1]);
-          context.styleSheets[0].addImport(match[1]);
         } while (match);
       }
     }
diff -u b/core/modules/ckeditor/js/ckeditor.js b/core/modules/ckeditor/js/ckeditor.js
--- b/core/modules/ckeditor/js/ckeditor.js
+++ b/core/modules/ckeditor/js/ckeditor.js
@@ -277,7 +277,7 @@
   CKEDITOR.timestamp = drupalSettings.ckeditor.timestamp;
 
   /**
-   * Command to add CSS to a CKEditor instance.
+   * Command to add CSS styles to a CKEditor instance.
    *
    * @param {Drupal.Ajax} [ajax]
    *   {@link Drupal.Ajax} object created by {@link Drupal.ajax}.
@@ -289,11 +289,35 @@
    *   The XMLHttpRequest status.
    */
-  Drupal.AjaxCommands.prototype.add_css_ckeditor = function (ajax, response, status) {
+  Drupal.AjaxCommands.prototype.ckeditor_add_style = function (ajax, response, status) {
     var editor = CKEDITOR.instances[response.editor_id];
+
+    if (editor) {
+      response.styles.forEach(function (style) {
+        editor.document.appendStyleText(style);
+      });
+    }
+  };
+
+  /**
+   * Command to add style sheets to a CKEditor instance.
+   *
+   * @param {Drupal.Ajax} [ajax]
+   *   {@link Drupal.Ajax} object created by {@link Drupal.ajax}.
+   * @param {object} response
+   *   The response from the Ajax request.
+   * @param {string} response.editor_id
+   *   The CKEditor instance ID.
+   * @param {number} [status]
+   *   The XMLHttpRequest status.
+   */
+  Drupal.AjaxCommands.prototype.ckeditor_add_stylesheet = function (ajax, response, status) {
+    var editor = CKEDITOR.instances[response.editor_id];
+
     if (editor) {
-      response.document = editor.document.$;
+      response.stylesheets.forEach(function (url) {
+        editor.document.appendStyleSheet(url);
+      });
     }
-    this.add_css.apply(this, arguments);
-  }
+  };
 
 })(Drupal, Drupal.debounce, CKEDITOR, jQuery);
diff -u b/core/modules/ckeditor/tests/modules/ckeditor_test.module b/core/modules/ckeditor/tests/modules/ckeditor_test.module
--- b/core/modules/ckeditor/tests/modules/ckeditor_test.module
+++ b/core/modules/ckeditor/tests/modules/ckeditor_test.module
@@ -7,6 +7,7 @@
 
 use Drupal\Core\Ajax\AjaxResponse;
 use Drupal\ckeditor\Ajax\AddCssCommand;
+use Drupal\ckeditor\Ajax\AddStylesCommand;
 use Drupal\editor\Entity\Editor;
 
 /**
@@ -23,12 +24,9 @@
 }
 
 function ckeditor_test_send_add_css_command() {
-  // Force all text in the editor to be red.
-  $css = '<style type="text/css">body { color: red !important; }</style>';
-
   return (new AjaxResponse)
     ->addCommand(
-      new AddCssCommand($css, 'edit-body-0-value')
+      new AddStylesCommand('edit-body-0-value', ['body { color: red; }'])
     );
 }
 
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/ckeditor/src/Ajax/AddStyleSheetCommand.php
@@ -0,0 +1,34 @@
+<?php
+
+namespace Drupal\ckeditor\Ajax;
+
+use Drupal\Core\Ajax\CommandInterface;
+
+class AddStyleSheetCommand implements CommandInterface {
+
+  protected $editorId;
+
+  protected $styleSheets = [];
+
+  public function __construct($editor_id, array $stylesheets = []) {
+    $this->editorId = $editor_id;
+    $this->styleSheets = $stylesheets;
+  }
+
+  public function addStyleSheet($stylesheet) {
+    $this->styleSheets[] = $stylesheet;
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function render() {
+    return [
+      'command' => 'ckeditor_add_stylesheet',
+      'editor_id' => $this->editorId,
+      'stylesheets' => $this->styleSheets,
+    ];
+  }
+
+}
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/ckeditor/src/Ajax/AddStylesCommand.php
@@ -0,0 +1,34 @@
+<?php
+
+namespace Drupal\ckeditor\Ajax;
+
+use Drupal\Core\Ajax\CommandInterface;
+
+class AddStylesCommand implements CommandInterface {
+
+  protected $editorId;
+
+  protected $styles = [];
+
+  public function __construct($editor_id, array $styles = []) {
+    $this->editorId = $editor_id;
+    $this->styles = $styles;
+  }
+
+  public function addStyle($style) {
+    $this->styles[] = $style;
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function render() {
+    return [
+      'command' => 'ckeditor_add_style',
+      'editor_id' => $this->editorId,
+      'styles' => $this->styles,
+    ];
+  }
+
+}