diff --git a/packages/docusaurus-cssnano-preset/src/index.ts b/packages/docusaurus-cssnano-preset/src/index.ts
index 6bae42c6f8c7..ddea9fc0ed9c 100644
--- a/packages/docusaurus-cssnano-preset/src/index.ts
+++ b/packages/docusaurus-cssnano-preset/src/index.ts
@@ -13,6 +13,8 @@ const preset: typeof advancedBasePreset = function preset(opts) {
   const advancedPreset = advancedBasePreset({
     autoprefixer: {add: false},
     discardComments: {removeAll: true},
+    /* cSpell:ignore zindex */
+    zindex: false,
     ...opts,
   });
 
diff --git a/website/_dogfooding/_pages tests/index.md b/website/_dogfooding/_pages tests/index.md
index cf94300fb891..d54f03fcd3f6 100644
--- a/website/_dogfooding/_pages tests/index.md	
+++ b/website/_dogfooding/_pages tests/index.md	
@@ -28,3 +28,4 @@ import Readme from "../README.md"
 - [General Markdown tests](/tests/pages/markdownPageTests)
 - [TOC tests](/tests/pages/page-toc-tests)
 - [Tabs tests](/tests/pages/tabs-tests)
+- [z-index tests](/tests/pages/z-index-tests)
diff --git a/website/_dogfooding/_pages tests/z-index-tests.tsx b/website/_dogfooding/_pages tests/z-index-tests.tsx
new file mode 100644
index 000000000000..d1d076b89495
--- /dev/null
+++ b/website/_dogfooding/_pages tests/z-index-tests.tsx	
@@ -0,0 +1,17 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import React from 'react';
+import Layout from '@theme/Layout';
+
+export default function zIndexTest(): JSX.Element {
+  return (
+    <Layout>
+      <p id="z-index-test">This should have a z-index of 100</p>
+    </Layout>
+  );
+}
diff --git a/website/_dogfooding/dogfooding.css b/website/_dogfooding/dogfooding.css
index be6cf970b6f9..f1864ef8fb82 100644
--- a/website/_dogfooding/dogfooding.css
+++ b/website/_dogfooding/dogfooding.css
@@ -20,3 +20,7 @@ html.plugin-blog.plugin-id-blog-tests .navbar {
 html.plugin-pages.plugin-id-pages-tests .navbar {
   border-bottom: solid thin yellow;
 }
+
+#z-index-test {
+  z-index: 100;
+}