Stray start tag “h1”–“h6”.

A heading, from <h1> to <h6>, that comes after a </body> or </html> end tag is stray, because the HTML parser accepts no more elements once the body has ended. The message names the heading level the validator found, and the cause and fix are the same for every level.

Once the parser reads </body>, only whitespace, comments and the closing </html> tag may follow. Any element after that point is a parse error. Browsers recover by treating the heading as part of the body again, so the page usually looks right and the mistake goes unnoticed. After </html> the validator also gives up: it reports "Cannot recover after last error" and checks nothing further down the page, so other errors stay hidden until you fix this one.

The </body> that ends the body is often not the one at the bottom of the page. A block of HTML included from a standalone file, such as a widget or a newsletter form, can bring its own <html>, <body>, </body> and </html> tags. The parser ignores the extra start tags, which the validator reports a few lines earlier, but it obeys the end tags, so the real body ends in the middle of the page. The heading that opens the next section is then the first element after that point. A template that closes the body before printing the rest of the content has the same effect.

Remove the <html>, <head> and <body> tags from the included block so only its content reaches the page, and keep one </body> and one </html> at the very end of the document. If the heading sits after the page's own </body>, move it inside the body.

Invalid example

<!DOCTYPE html>
<html lang="en">
<head>
  <title>News</title>
</head>
<body>
  <main>
    <h1>News</h1>
    <!-- Included newsletter block -->
    <html>
    <body>
      <p>Subscribe to our newsletter.</p>
    </body>
    </html>
    <h2>Latest articles</h2>
    <p>New articles appear here every week.</p>
  </main>
</body>
</html>

Valid example

<!DOCTYPE html>
<html lang="en">
<head>
  <title>News</title>
</head>
<body>
  <main>
    <h1>News</h1>
    <!-- Included newsletter block -->
    <p>Subscribe to our newsletter.</p>
    <h2>Latest articles</h2>
    <p>New articles appear here every week.</p>
  </main>
</body>
</html>

Learn more:

MDN Reference

MDN Web Docs: Heading elements

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MDN Reference

MDN Web Docs: <body>

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WHATWG Spec

HTML Standard: Parsing HTML documents

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