diff --git a/src/extension-support/tw-iframe-extension-worker.js b/src/extension-support/tw-iframe-extension-worker.js
index 9eccebba..c57ac917 100644
--- a/src/extension-support/tw-iframe-extension-worker.js
+++ b/src/extension-support/tw-iframe-extension-worker.js
@@ -53,7 +53,8 @@ class IframeExtensionWorker {
 
         window.addEventListener('message', this._onWindowMessage.bind(this));
         const blob = new Blob([
-            `<body><script>window.__WRAPPED_IFRAME_ID__=${JSON.stringify(this.id)};${frameSource}</script></body>`
+            // eslint-disable-next-line max-len
+            `<!DOCTYPE html><body><script>window.__WRAPPED_IFRAME_ID__=${JSON.stringify(this.id)};${frameSource}</script></body>`
         ], {
             type: 'text/html'
         });