vercel · Timer · Jul 7, 2020 · May 27, 2020 · May 27, 2020 · May 28, 2020
diff --git a/errors/no-document-title.md b/errors/no-document-title.md
@@ -10,34 +10,21 @@ Set `<title>` in `pages/_app.js` instead:
 
 ```js
 // pages/_app.js
-import App from 'next/app'
-import Head from 'next/head'
 import React from 'react'
+import Head from 'next/head'
 
-export default class MyApp extends App {
-  static async getInitialProps({ Component, ctx }) {
-    let pageProps = {}
-
-    if (Component.getInitialProps) {
-      pageProps = await Component.getInitialProps(ctx)
-    }
-
-    return { pageProps }
-  }
-
-  render() {
-    const { Component, pageProps } = this.props
-
-    return (
-      <>
-        <Head>
-          <title>My new cool app</title>
-        </Head>
-        <Component {...pageProps} />
-      </>
-    )
-  }
+function MyApp({ Component, pageProps }) {
+  return (
+    <>
+      <Head>
+        <title>My new cool app</title>
+      </Head>
+      <Component {...pageProps} />
+    </>
+  )
 }
+
+export default MyApp
 ```
 
 ### Useful Links

diff --git a/errors/no-document-viewport-meta.md b/errors/no-document-viewport-meta.md
@@ -0,0 +1,33 @@
+# Viewport `meta` tags should not be used in `_document.js`'s `<Head>`
+
+#### Why This Error Occurred
+
+Adding `<meta name="viewport" ...>` in `pages/_document.js` will lead to unexpected results since it cannot be deduped.
+The viewport tag should be handled by `next/head` in `pages/_app.js`.
+
+#### Possible Ways to Fix It
+
+Set your viewport `meta` tag in `pages/_app.js` instead:
+
+```jsx
+// pages/_app.js
+import React from 'react'
+import Head from 'next/head'
+
+function MyApp({ Component, pageProps }) {
+  return (
+    <>
+      <Head>
+        <meta name="viewport" content="viewport-fit=cover" />
+      </Head>
+      <Component {...pageProps} />
+    </>
+  )
+}
+
+export default MyApp
+```
+
+### Useful Links
+
+- [Issue #13230](https://github.com/zeit/next.js/issues/13230), which led to the creation of this warning.
diff --git a/packages/next/pages/_document.tsx b/packages/next/pages/_document.tsx
@@ -256,10 +256,19 @@ export class Head extends Component<
     if (process.env.NODE_ENV !== 'production') {
       children = React.Children.map(children, (child: any) => {
         const isReactHelmet = child?.props?.['data-react-helmet']
-        if (child?.type === 'title' && !isReactHelmet) {
-          console.warn(
-            "Warning: <title> should not be used in _document.js's <Head>. https://err.sh/next.js/no-document-title"
-          )
+        if (!isReactHelmet) {
+          if (child?.type === 'title') {
+            console.warn(
+              "Warning: <title> should not be used in _document.js's <Head>. https://err.sh/next.js/no-document-title"
+            )
+          } else if (
+            child?.type === 'meta' &&
+            child?.props?.name === 'viewport'
+          ) {
+            console.warn(
+              "Warning: viewport meta tags should not be used in _document.js's <Head>. https://err.sh/next.js/no-document-viewport-meta"
+            )
+          }
         }
         return child
       })

diff --git a/test/integration/document-head-warnings/pages/_document.js b/test/integration/document-head-warnings/pages/_document.js
@@ -0,0 +1,20 @@
+import Document, { Html, Head, Main, NextScript } from 'next/document'
+
+class MyDocument extends Document {
+  render() {
+    return (
+      <Html>
+        <Head crossOrigin="anonymous">
+          <title>Check out this title!</title>
+          <meta name="viewport" content="viewport-fit=cover" />
+        </Head>
+        <body>
+          <Main />
+          <NextScript />
+        </body>
+      </Html>
+    )
+  }
+}
+
+export default MyDocument
diff --git a/test/integration/document-head-warnings/pages/index.js b/test/integration/document-head-warnings/pages/index.js
@@ -0,0 +1,14 @@
+function Hi() {
+  return (
+    <div>
+      <p>Hello world!</p>
+      <style jsx>{`
+        p {
+          font-size: 16.4px;
+        }
+      `}</style>
+    </div>
+  )
+}
+
+export default Hi
diff --git a/test/integration/document-head-warnings/test/index.test.js b/test/integration/document-head-warnings/test/index.test.js
@@ -0,0 +1,43 @@
+/* eslint-env jest */
+
+import { join } from 'path'
+import { renderViaHTTP, findPort, launchApp, killApp } from 'next-test-utils'
+
+jest.setTimeout(1000 * 60)
+const appDir = join(__dirname, '..')
+let output
+
+describe('Custom Document Head Warnings', () => {
+  beforeAll(async () => {
+    const handleOutput = (msg) => {
+      output += msg
+    }
+    const appPort = await findPort()
+    const app = await launchApp(appDir, appPort, {
+      onStdout: handleOutput,
+      onStderr: handleOutput,
+    })
+    await renderViaHTTP(appPort, '/')
+    await killApp(app)
+  })
+
+  describe('development mode', () => {
+    it('warns when using a <title> in document/head', () => {
+      expect(output).toMatch(
+        /.*Warning: <title> should not be used in _document.js's <Head>\..*/
+      )
+    })
+
+    it('warns when using viewport meta tags in document/head', () => {
+      expect(output).toMatch(
+        /.*Warning: viewport meta tags should not be used in _document.js's <Head>\..*/
+      )
+    })
+
+    it('warns when using a crossOrigin attribute on document/head', () => {
+      expect(output).toMatch(
+        /.*Warning: `Head` attribute `crossOrigin` is deprecated\..*/
+      )
+    })
+  })
+})