Missing canonical URL
This means that the URL in question has hreflang annotations, but no canonical tag.
Why is this important?
URLs with hreflang do not need to have canonical tags - so it is NOT a problem if your site does not use them.
However, it is worth considering that canonicals and hreflang are both indexing instructions. If you can give more precise, consistent indexing signals to search engines, not only will their indexing and linking properties be more accurate, but they will be better able to serve users the URL of their preferred language.
What does the Hint check?
This Hint will trigger for any URL which has hreflang annotations, but no canonical tag.
Examples that trigger this Hint:
Consider the URL: https://example.com/en/page-a/
The Hint would trigger for this URL if it did have hreflang, but was missing a canonical in the <head>:
AND if there was also no canonical present in the HTTP header.
Why is this Hint marked 'Advisory'?
This Hint is marked Advisory as it does not represent a clear issue. It is not necessary for hreflang URLs to include a canonical, but it could be considered helpful to include one.
However, if you using hreflang in conjunction with canonicals, ensure that the canonicals are self-referencing.