diff --git a/package.json b/package.json
index c601072126..c3cd054db8 100644
--- a/package.json
+++ b/package.json
@@ -76,7 +76,7 @@
     "focus-visible": "^5.2.0",
     "gfm.css": "^1.1.2",
     "glob-to-regexp": "^0.4.1",
-    "highlight.js": "^10.5.0",
+    "highlight.js": "^11.3.1",
     "html-entities": "^1.4.0",
     "is-ip": "^3.1.0",
     "jszip": "^3.7.0",
@@ -188,7 +188,9 @@
     "@types/react": "17.0.14"
   },
   "jest": {
-    "snapshotSerializers": ["enzyme-to-json/serializer"],
+    "snapshotSerializers": [
+      "enzyme-to-json/serializer"
+    ],
     "testEnvironment": "./__test-utils__/environment.js",
     "testMatch": [
       "<rootDir>/test/**/*-test.[jt]s?(x)"
diff --git a/res/css/views/rooms/_EventTile.scss b/res/css/views/rooms/_EventTile.scss
index cde3e767f6..5559f81e0a 100644
--- a/res/css/views/rooms/_EventTile.scss
+++ b/res/css/views/rooms/_EventTile.scss
@@ -500,12 +500,11 @@ $hover-select-border: 4px;
     float: left;
     margin: 0 0.5em 0 -1.5em;
     color: gray;
-}
-
-.mx_EventTile_lineNumber {
-    text-align: right;
-    display: block;
-    padding-left: 1em;
+    & span {
+        text-align: right;
+        display: block;
+        padding-left: 1em;
+    }
 }
 
 .mx_EventTile_collapsedCodeBlock {
diff --git a/src/components/views/messages/TextualBody.tsx b/src/components/views/messages/TextualBody.tsx
index 63ff39721d..503105b9db 100644
--- a/src/components/views/messages/TextualBody.tsx
+++ b/src/components/views/messages/TextualBody.tsx
@@ -45,6 +45,8 @@ import EditMessageComposer from '../rooms/EditMessageComposer';
 import LinkPreviewGroup from '../rooms/LinkPreviewGroup';
 import { IBodyProps } from "./IBodyProps";
 
+const MAX_HIGHLIGHT_LENGTH = 4096;
+
 interface IState {
     // the URLs (if any) to be previewed with a LinkPreviewWidget inside this TextualBody.
     links: string[];
@@ -117,9 +119,6 @@ export default class TextualBody extends React.Component<IBodyProps, IState> {
                 setTimeout(() => {
                     if (this.unmounted) return;
                     for (let i = 0; i < codes.length; i++) {
-                        // If the code already has the hljs class we want to skip this.
-                        // This happens after the codeblock was edited.
-                        if (codes[i].className.includes("hljs")) continue;
                         this.highlightCode(codes[i]);
                     }
                 }, 10);
@@ -212,30 +211,54 @@ export default class TextualBody extends React.Component<IBodyProps, IState> {
     private addLineNumbers(pre: HTMLPreElement): void {
         // Calculate number of lines in pre
         const number = pre.innerHTML.replace(/\n(<\/code>)?$/, "").split(/\n/).length;
-        pre.innerHTML = '<span class="mx_EventTile_lineNumbers"></span>' + pre.innerHTML + '<span></span>';
-        const lineNumbers = pre.getElementsByClassName("mx_EventTile_lineNumbers")[0];
+        const lineNumbers = document.createElement('span');
+        lineNumbers.className = 'mx_EventTile_lineNumbers';
         // Iterate through lines starting with 1 (number of the first line is 1)
         for (let i = 1; i <= number; i++) {
-            lineNumbers.innerHTML += '<span class="mx_EventTile_lineNumber">' + i + '</span>';
+            const s = document.createElement('span');
+            s.textContent = i.toString();
+            lineNumbers.appendChild(s);
         }
+        pre.prepend(lineNumbers);
+        pre.append(document.createElement('span'));
     }
 
     private highlightCode(code: HTMLElement): void {
-        // Auto-detect language only if enabled and only for codeblocks
-        if (
+        if (code.textContent.length > MAX_HIGHLIGHT_LENGTH) {
+            console.log(
+                "Code block is bigger than highlight limit (" +
+                code.textContent.length + " > " + MAX_HIGHLIGHT_LENGTH +
+                "): not highlighting",
+            );
+            return;
+        }
+        console.log('highlighting');
+
+        let advertisedLang;
+        for (const cl of code.className.split(/\s+/)) {
+            if (cl.startsWith('language-')) {
+                const maybeLang = cl.split('-', 2)[1];
+                if (highlight.getLanguage(maybeLang)) {
+                    advertisedLang = maybeLang;
+                    break;
+                }
+            }
+        }
+
+        if (advertisedLang) {
+            // If the code says what language it is, highlight it in that language
+            // We don't use highlightElement here because we can't force language detection
+            // off. It should use the one we've found in the CSS class but we'd rather pass
+            // it in explicitly to make sure.
+            code.innerHTML = highlight.highlight(advertisedLang, code.textContent).value;
+        } else if (
             SettingsStore.getValue("enableSyntaxHighlightLanguageDetection") &&
             code.parentElement instanceof HTMLPreElement
         ) {
-            highlight.highlightBlock(code);
-        } else {
-            // Only syntax highlight if there's a class starting with language-
-            const classes = code.className.split(/\s+/).filter(function(cl) {
-                return cl.startsWith('language-') && !cl.startsWith('language-_');
-            });
-
-            if (classes.length != 0) {
-                highlight.highlightBlock(code);
-            }
+            // User has language detection enabled and the code is within a pre
+            // we only auto-highlight if the code block is in a pre), so highlight
+            // the block with auto-highlighting enabled.
+            highlight.highlightElement(code);
         }
     }
 
diff --git a/yarn.lock b/yarn.lock
index 0b325d66d9..0404e9b6b7 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -4350,10 +4350,10 @@ has@^1.0.0, has@^1.0.1, has@^1.0.3:
   dependencies:
     function-bind "^1.1.1"
 
-highlight.js@^10.5.0:
-  version "10.7.3"
-  resolved "https://registry.yarnpkg.com/highlight.js/-/highlight.js-10.7.3.tgz#697272e3991356e40c3cac566a74eef681756531"
-  integrity sha512-tzcUFauisWKNHaRkN4Wjl/ZA07gENAjFl3J/c480dprkGTg5EQstgaNFqBfUqCq54kZRIEcreTsAgF/m2quD7A==
+highlight.js@^11.3.1:
+  version "11.3.1"
+  resolved "https://registry.yarnpkg.com/highlight.js/-/highlight.js-11.3.1.tgz#813078ef3aa519c61700f84fe9047231c5dc3291"
+  integrity sha512-PUhCRnPjLtiLHZAQ5A/Dt5F8cWZeMyj9KRsACsWT+OD6OP0x6dp5OmT5jdx0JgEyPxPZZIPQpRN2TciUT7occw==
 
 hoist-non-react-statics@^3.3.0, hoist-non-react-statics@^3.3.2:
   version "3.3.2"