Canonical Tag - Mobile or Desktop Version Teller
This is important for telling the search engine to mobile and desktop versions. It is used in the head tag. It is not itself a tag but used with rel canonical and rel alternate.
Canonical Tag: In the image, you are instructed about the truth about the URL status.
Only HTTP 200 status code can be used to rel canonical.
Moreover, the canonical URL should be similar or identical that you can say in other words mobile version and desktop version.
My post about mobile SEO has detail about how to use this tag.
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