Canonical Tag May Be Ignored If Pages Aren’t Equivalent
Google will ignore canonical tags where the target page and canonicalized page are different. Canonicalization is meant to be used for identical or equivalent pages only.
Internally Linked Parameter URLs Might be Indexed Even if Canonical Points to Clean Version
When using parameter URLs for internal linking, it isn’t always guaranteed that the clean version of the URL be indexed if there is a canonical tag to this version. This is because Google has to weigh up which URL should be shown in search.
There’s No Recommended Size for ‘View All’ Pages to Be Chosen as Canonical
A ‘view all’ page doesn’t have to have a specific number of products or be a certain size in order for it to be accepted as the canonical for other pages.
Passing Signals Within a Large Group of Canonicalized Pages Takes Time
If a large group of pages is included within a canonicalized set, Google will still have to crawl and process all of these pages to find the canonical and pass on any link equity if the canonicalised pages are linked to externally.
Canonicalizing Paginated Pages Back to Main Page Can Cause Crawling & Indexing Issues
Canonicalizing pages in a paginated set back to the first page can be problematic because Google may see that these are different pages and ignore the rel canonical. Alternatively, if Google does follow the rel canonical to the main page, this could mean links and content on other pages might be missed.
Hreflang is a Minor Canonicalization Signal
Google uses hreflang as a small signal when picking the canonical version of a page, but this needs to be backed up by consistent signals provided by rel canonicals, internal linking and sitemap files.
Implement Clear Canonical Version Per Country & Hreflang Between Equivalent Canonical Versions
Don’t specify a canonical between different country versions of a page, because Google will likely only index the preferred version. John recommends having a clear canonical version per country and implementing hreflang between the canonical versions for each country.
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.
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.