diff --git a/src_js/components/PreviewDiffButton.tsx b/src_js/components/PreviewDiffButton.tsx
new file mode 100644
index 0000000..4e58eb0
--- /dev/null
+++ b/src_js/components/PreviewDiffButton.tsx
@@ -0,0 +1,81 @@
+import { Fragment, h } from 'preact';
+
+const PRIMER_SPEC_PREVIEW_HOSTNAME = 'preview.sesh.rs';
+const GITHUB_REPO_REGEX_FROM_PREVIEW_URL =
+  /^https:\/\/preview\.sesh\.rs\/previews\/([A-Za-z0-9_-]+)\/([A-Za-z0-9_-]+)\/\d+\/(.*)/;
+
+const HTML_DIFF_URL = 'https://services.w3.org/htmldiff';
+
+export function PreviewDiffButton() {
+  if (window.location.hostname !== PRIMER_SPEC_PREVIEW_HOSTNAME) {
+    return null;
+  }
+
+  const productionUrl = getProductionUrl();
+  if (!productionUrl) {
+    return null;
+  }
+
+  return (
+    <Fragment>
+      <style>
+        {'.btn-primer-spec-preview {'}
+        {'  position: fixed;'}
+        {'  top: 15%;'}
+        {'  right: 1em;'}
+        {'  transition: width 0.5s !important;'}
+        {'  width: 3.5em;'}
+        {'}'}
+        {'.btn-primer-spec-preview:hover {'}
+        {'  width: 22em;'}
+        {'}'}
+        {'.primer-spec-preview-show-on-hover {'}
+        {'  opacity: 0;'}
+        {'  /* Transition applies onMouseOut (text disappears faster) */'}
+        {'  transition: opacity 0.15s;'}
+        {'}'}
+        {'.btn-primer-spec-preview:hover .primer-spec-preview-show-on-hover {'}
+        {'  opacity: 1;'}
+        {'  /* Transition applies onMouseOver (hence we add a delay) */'}
+        {'  transition: opacity 0.3s 0.3s;'}
+        {'}'}
+      </style>
+      <button
+        class="btn btn-primary btn-primer-spec-preview"
+        onClick={() => {
+          window.open(buildDiffUrl(productionUrl), '_blank');
+        }}
+      >
+        <i class="fas fa-glasses" style="font-weight: 900; opacity: 1;" />{' '}
+        <span class="primer-spec-preview-show-on-hover">
+          Compare preview with published page
+        </span>
+      </button>
+    </Fragment>
+  );
+}
+
+function getProductionUrl() {
+  // const matches = window.location.href.match(
+  //   GITHUB_REPO_REGEX_FROM_PREVIEW_URL,
+  // );
+  const matches =
+    'https://preview.sesh.rs/previews/eecs485staff/p1-insta485-static/492/setup_macos.html'.match(
+      GITHUB_REPO_REGEX_FROM_PREVIEW_URL,
+    );
+  if (matches && matches.length >= 4) {
+    const org = matches[1];
+    const repo = matches[2];
+    const path = matches[3];
+    return `https://${org}.github.io/${repo}/${path}`;
+  }
+  return null;
+}
+
+function buildDiffUrl(publishedUrl: string) {
+  const queryParams = new URLSearchParams({
+    doc1: publishedUrl,
+    doc2: window.location.href,
+  });
+  return `${HTML_DIFF_URL}?${queryParams}`;
+}
diff --git a/src_js/components/PrimerSpec.tsx b/src_js/components/PrimerSpec.tsx
index 9ea4943..b9d9113 100644
--- a/src_js/components/PrimerSpec.tsx
+++ b/src_js/components/PrimerSpec.tsx
@@ -11,6 +11,7 @@ import { useAfterPrint, useBeforePrint } from '../utils/hooks/print';
 import useSmallScreen from '../utils/hooks/useSmallScreen';
 import Config from '../Config';
 import MainContent from './main_content';
+import { PreviewDiffButton } from './PreviewDiffButton';
 import Settings from './settings';
 import Sidebar from './sidebar';
 import Topbar from './Topbar';
@@ -150,6 +151,7 @@ export default function PrimerSpec(props: PropsType): h.JSX.Element {
         onSubthemeNameChange={(name) => setTheme({ name })}
         onSubthemeModeChange={(mode) => setTheme({ mode })}
       />
+      <PreviewDiffButton />
     </Fragment>
   );
 }