Canonical mismatch between rendered and response HTML
This means that the URL in question contains a canonical element has a different link in the response HTML, when compared with the rendered HTML.
Why is this important?
Rendering webpages is a resource intensive task, and it takes significantly longer than simply grabbing source (response) HTML content.
This is why Google essentially crawl URLs in a two-stage process: their 'first look' is of the HTML response, then they render the page and have a 'second look' at the rendered HTML, then they update the index based on what they found in the rendered HTML.
As such, it is unwise to provide differing indexing signals in the response and rendered HTML (i.e. robots, canonicals,hreflang, etc...)
What does the Hint check?
This Hint will trigger for any internal URL that contains a canonical link element that is different in the rendered HTML than it is in the response HTML.
Examples that trigger this Hint
This Hint will trigger for any URL that has a canonical link element in the <head> of the response HTML, with a different link in the rendered HTML.
For example, if the response HTML looked like this:
And the rendered HTML looked like this:
How do you resolve this issue?
Google have stated categorically that the rendered canonical is not taken into account, so relying on it for indexing purposes would be unwise.
Google do caveat their statement by saying, 'if you really have to do it, then do it this way':