Missing self-reference hreflang annotation
This means that the URL in question has no self-referenced hreflang annotation.
Why is this important?
Self-reference hreflang is not actually a mandatory requirement. It appeared in the original recommendations that Google published as they figured it would be easier for users to just copy/paste the same 'hreflang cluster' on all corresponding translation pages.
So it is NOT a problem if your site does not use it.
However, some website owners find it easier to manage hreflang when it does include self-referenced annotations.
What does the Hint check?
This Hint will trigger for any URL which has no self-referenced hreflang annotation, using any method (HTML, HTTP header, or XML Sitemap).
Examples that trigger this Hint:
Consider the URL: https://example.com/en/page-a/
The Hint would trigger for this URL if it was missing the following hreflang annotation in the <head>:
AND if this self-reference was not present in the HTTP header, or in an XML Sitemap.
Why is this Hint marked 'Advisory'?
This Hint is marked Advisory as it does not represent a clear issue. It is not necessary for URLs to include a hreflang self-reference, but it could be considered helpful to include one.
If you did want to include self-referenced hreflang, you'd need to manually update the pages to add them in, or fix the script that generates the hreflang (depending on how the site is set up).