This means that the URL in question is an AMP Page URL, which does not contain an <html amp> tag.
AMP HTML documents are required to contain a top-level <html amp> tag. If the tag is not present, search engines may not consider the HTML valid and may not index the page properly, or include it in search results.
This Hint will trigger for any AMP Page that does not include the <html amp> tag.
Consider the AMP URL: https://example.com/page-a/amp/
The Hint would trigger for this URL if it had no <html amp> or <html ⚡> tag in the HTML.
Add a <html amp> tag in the <head> (<html ⚡> can also be used, interchangeably):
<!doctype html>
<html amp>
<head>
<meta charset="utf-8">
<title>Sample document</title>
...
</head>
...
</html>
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