JavaScript Injected Tags Should Not be Duplicated in Static HTML
Using JavaScript to modify canonical or robots meta tags can change the signal provided to Google when they come to process the rendered version of the page. If tags are injected using JavaScript, then John recommends not having them in the static HTML so that the signal provided to Google is clear.
Google Doesn’t Support Canonical Tags for Images
Canonical tags don’t work for images, so you can’t fold together different sizes of an image to make sure only the best size is indexed, for example. Google is looking into how best to use srcset for image search, however.
Canonical Selection Doesn’t Affect Rankings For a Group of Pages
Google will simply show one URL out of a group of pages that have been canonicalised. The ranking will be the same for that group of URLs no matter which one Google decides to show.
Different Signals Determine Google’s Canonical Selection
John confirmed that rel canonical, redirects, internal linking, URL parameters and sitemaps are all signals Google uses to decide which page is the canonical from a group of pages that have been folded together.
Canonicalise Duplicate Pages Between Your Sites so They’re Not Seen as Doorway Pages
Use the canonical tag if you are offering the same products on lots of different sites so Google doesn’t suspect that these are doorway pages.
Make Sure A Separate PWA is Canonicalised to Your Mobile Site
Google won’t get confused between your main site and separate PWA as long as you have canonicalization in place.
Internal Linking Causes Google to Crawl Canonicalised Pages
Check your internal linking if you see Google crawling canonicalised pages.
Hreflang Should be Included Between the Canonical Versions of Pages
Including hreflang on paginated pages is fine but the most important thing is to include hreflang tags between the canonical versions of the page.
Parameter Handling Signals Are Stronger Than the Canonical Signal
Google won’t blindly follow URL parameter handling set in GSC, but John says that this is a stronger signal than using canonicals.