diff --git a/extension/background.ts b/extension/background.ts
index aad09f7b5..fa7f1d0f2 100644
--- a/extension/background.ts
+++ b/extension/background.ts
@@ -32,6 +32,19 @@ chrome.runtime.onMessage.addListener(async (request, sender, response) => {
       }
       rcxMain.onTabSelect(sender.tab.id);
       break;
+    case 'forceDocsHtml?':
+      console.log('forceDocsHtml?');
+      if (rcxMain.enabled === 1) {
+        response(true);
+        chrome.tabs.sendMessage(sender.tab!.id!, {
+          type: 'showPopup',
+          text: `
+            rikaikun is forcing Google Docs to render using HTML instead of canvas.<br>
+            rikaikun can't work with canvas mode but if you need that mode, please disable rikaikun.
+          `,
+        });
+      }
+      break;
     case 'xsearch':
       console.log('xsearch');
       response(rcxMain.search(request.text, request.dictOption));
@@ -70,3 +83,5 @@ chrome.runtime.onMessage.addListener(async (request, sender, response) => {
 // Clear browser action badge text on first load
 // Chrome preserves last state which is usually 'On'
 chrome.browserAction.setBadgeText({ text: '' });
+
+export { rcxMainPromise as TestOnlyRxcMainPromise };
diff --git a/extension/docs-html-fallback.ts b/extension/docs-html-fallback.ts
new file mode 100644
index 000000000..9881838aa
--- /dev/null
+++ b/extension/docs-html-fallback.ts
@@ -0,0 +1,23 @@
+function forceHtml(force: boolean) {
+  if (!force) {
+    return;
+  }
+  console.log(
+    'rikaikun is forcing Docs to use HTML instead of canvas for rendering.'
+  );
+  const injectedCode = `(function() {window['_docs_force_html_by_ext'] = '${chrome.runtime.id}';})();`;
+
+  const script = document.createElement('script');
+
+  script.textContent = injectedCode;
+
+  // Usually, `document.head` isn't guaranteed to be present when content_scripts run but in this case
+  // we're running inside a callback so it should be 100% safe.
+  document.head.appendChild(script);
+}
+
+// This check allows the user to get newer Docs Canvas without disabling rikaikun.
+// This delays when the forcing code is injected but it seems to be early enough in practice.
+chrome.runtime.sendMessage({ type: 'forceDocsHtml?' }, forceHtml);
+
+export { forceHtml as TestOnlyForceHtml };
diff --git a/extension/manifest.json b/extension/manifest.json
index 6f9852a89..39885c532 100644
--- a/extension/manifest.json
+++ b/extension/manifest.json
@@ -26,6 +26,12 @@
       "match_about_blank": true,
       "js": ["rikaicontent.js"],
       "all_frames": true
+    },
+    {
+      "matches": ["https://docs.google.com/*"],
+      "js": ["docs-html-fallback.js"],
+      "run_at": "document_start",
+      "all_frames": true
     }
   ],
   "web_accessible_resources": [
diff --git a/extension/test/background_test.ts b/extension/test/background_test.ts
new file mode 100644
index 000000000..7ea81784e
--- /dev/null
+++ b/extension/test/background_test.ts
@@ -0,0 +1,88 @@
+import { RcxMain } from '../rikaichan';
+import { expect, use } from '@esm-bundle/chai';
+import chrome from 'sinon-chrome';
+import sinon from 'sinon';
+import sinonChai from 'sinon-chai';
+
+use(sinonChai);
+
+let rcxMain: RcxMain;
+
+describe('background.ts', () => {
+  before(async () => {
+    // Resolve config fetch with minimal config object.
+    chrome.storage.sync.get.yields({ kanjiInfo: [] });
+    // Imports only run once so run in `before` to make it deterministic.
+    rcxMain = await (await import('../background')).TestOnlyRxcMainPromise;
+  });
+
+  beforeEach(() => {
+    // Only reset the spies we're using since we need to preserve
+    // the state of `chrome.runtime.onMessage.addListener` for invoking
+    // the core functionality of background.ts.
+    chrome.tabs.sendMessage.reset();
+  });
+
+  describe('when sent "forceDocsHtml?" message', () => {
+    it('should not call response callback when rikaikun disabled', async () => {
+      rcxMain.enabled = 0;
+      const responseCallback = sinon.spy();
+
+      await sendMessageToBackground({
+        type: 'forceDocsHtml?',
+        responseCallback: responseCallback,
+      });
+
+      expect(responseCallback).to.have.not.been.called;
+    });
+
+    it('should not send "showPopup" message when rikaikun disabled', async () => {
+      rcxMain.enabled = 0;
+
+      await sendMessageToBackground({
+        type: 'forceDocsHtml?',
+      });
+
+      expect(chrome.tabs.sendMessage).to.have.not.been.called;
+    });
+
+    it('should pass true to response callback when rikaikun enabled', async () => {
+      rcxMain.enabled = 1;
+      const responseCallback = sinon.spy();
+
+      await sendMessageToBackground({
+        type: 'forceDocsHtml?',
+        responseCallback: responseCallback,
+      });
+
+      expect(responseCallback).to.have.been.calledOnceWith(true);
+    });
+
+    it('should send "showPopup" message when rikaikun enabled', async () => {
+      rcxMain.enabled = 1;
+
+      await sendMessageToBackground({
+        type: 'forceDocsHtml?',
+      });
+
+      expect(chrome.tabs.sendMessage).to.have.been.calledWith(
+        /* tabId= */ sinon.match.any,
+        sinon.match({ type: 'showPopup' })
+      );
+    });
+  });
+});
+
+async function sendMessageToBackground({
+  type,
+  responseCallback = () => {},
+}: {
+  type: string;
+  responseCallback?: Function;
+}): Promise<void> {
+  return await chrome.runtime.onMessage.addListener.yield(
+    { type: type },
+    { tab: { id: 0 } },
+    responseCallback
+  );
+}
diff --git a/extension/test/docs-html-fallback_test.ts b/extension/test/docs-html-fallback_test.ts
new file mode 100644
index 000000000..75987a54a
--- /dev/null
+++ b/extension/test/docs-html-fallback_test.ts
@@ -0,0 +1,41 @@
+//import '../background';
+import { expect, use } from '@esm-bundle/chai';
+import chrome from 'sinon-chrome';
+
+declare global {
+  interface Window {
+    _docs_force_html_by_ext?: string;
+  }
+}
+
+let forceHtmlCallback: (force: boolean) => void;
+
+describe('docs-html-fallback.ts after sending `forceDocsHtml?` message', () => {
+  before(async () => {
+    await import('../docs-html-fallback');
+    forceHtmlCallback = chrome.runtime.sendMessage.args[0][1];
+  });
+
+  beforeEach(() => {
+    chrome.reset();
+    window._docs_force_html_by_ext = undefined;
+  });
+
+  describe('when `forceHtml` callback is called with `false`', () => {
+    it('should not add special property to window object', async () => {
+      forceHtmlCallback(false);
+
+      expect(window._docs_force_html_by_ext).to.be.undefined;
+    });
+  });
+
+  describe('when `forceHtml` callback is called with `true`', () => {
+    it('should set special property to rikaikun extension ID', async () => {
+      chrome.runtime.id = 'test_special_id';
+
+      forceHtmlCallback(true);
+
+      expect(window._docs_force_html_by_ext).to.equal(chrome.runtime.id);
+    });
+  });
+});
diff --git a/snowpack.config.js b/snowpack.config.js
index 6a4beaf5c..3997996c9 100644
--- a/snowpack.config.js
+++ b/snowpack.config.js
@@ -15,7 +15,7 @@ const config = {
         list: [
           //Remove test only export from rikaicontent
           {
-            from: /export.*TestOnlyRcxContent.*\n/,
+            from: /export.*TestOnly.*\n/,
             to: '',
           },
         ],