Hreflang Not Necessary For Non-Canonical Pages
It isn’t necessary to implement hreflang on the non-canonical version of a page because it won’t be shown in search, therefore any hreflang annotation won’t be used by Google.
Canonical Tag Can be on Either AMP Page or Legacy Page within an AMP HTML Pair
If you have the rel=amphtml tag set up correctly, Google can pick up the canonical tag from either page in the pairing.
Self-referencing Canonical Tags Are Best Practice But Not Critical
It’s best practice to have self-referencing canonical tags as the canonical tag is one of the signals that Google uses for selecting the primary page in a group of detected duplicates, however, it is not essential.
Google Will Use Other Canonicalization Factors If the Canonical Is Noindex
Google would receive conflicting signals if a canonical points to a noindex page. John suggested that Google would rely on other canonicalization factors in this scenario to decide which page should be indexed, such as internal links.
Soft 404s Cannot Be Passed to Other Pages via Redirects or Canonicals
A soft 404 cannot be passed on to another page via a canonical or redirect. Google ignores the content on a page if it detects that it is a 404 or soft 404.
Use Hreflang & Canonical Tags to Handle Partially Translated Websites
John recommends using canonical tags and hreflang tags to tell Google which language version is preferred if you only have parts of a website that are translated, instead of the entire site.
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.