diff --git a/ext/js/core/utilities.js b/ext/js/core/utilities.js
index ec29a3036..2e0386fee 100644
--- a/ext/js/core/utilities.js
+++ b/ext/js/core/utilities.js
@@ -262,3 +262,13 @@ export function deferPromise() {
 export function promiseTimeout(delay) {
     return delay <= 0 ? Promise.resolve() : new Promise((resolve) => { setTimeout(resolve, delay); });
 }
+
+/**
+ * @param {string} css
+ * @returns {string}
+ */
+export function sanitizeCSS(css) {
+    const sanitizer = new CSSStyleSheet();
+    sanitizer.replaceSync(css);
+    return Array.from(sanitizer.cssRules).map(rule => rule.cssText || '').join('\n');
+}
diff --git a/ext/js/data/anki-note-builder.js b/ext/js/data/anki-note-builder.js
index 11f6ea041..0bf592205 100644
--- a/ext/js/data/anki-note-builder.js
+++ b/ext/js/data/anki-note-builder.js
@@ -17,7 +17,7 @@
  */
 
 import {ExtensionError} from '../core/extension-error.js';
-import {deferPromise} from '../core/utilities.js';
+import {deferPromise, sanitizeCSS} from '../core/utilities.js';
 import {convertHiraganaToKatakana, convertKatakanaToHiragana} from '../language/ja/japanese.js';
 import {cloneFieldMarkerPattern, getRootDeckName} from './anki-util.js';
 
@@ -192,7 +192,7 @@ export class AnkiNoteBuilder {
         for (const dictionary of dictionaries) {
             const {name, styles} = dictionary;
             if (typeof styles === 'string') {
-                styleMap.set(name, styles);
+                styleMap.set(name, sanitizeCSS(styles));
             }
         }
         return styleMap;