Google’s John Mueller reiterated his previous advice that m-dot URLs should not be changed to canonical URLs even when Google fully transitions to mobile-first indexing. He said that this is simply the way it has always been done, and that switching on Google’s side would cause problems for many large sites.
Just to be clear, Google stated in 2017: “Interlinking with alternative mobile URLs (m.-dot sites) requires no changes. For sites that use alternative mobile URLs, please maintain the existing link rel=canonical and link rel=alternate elements across those versions.”
John also repeated this statement on LinkedIn: “Since Google indexes the mobile version of the URL but not the desktop version, should sites with m-dot URLs switch to canonicalizing to the mobile version now? Summary: No, don’t change it.”
And he explained why: simply put, it’s always been done the other way around, and changing it would cause a lot of problems for very large sites.
That might make sense: if Google is choosing mobile URLs as canonical, shouldn’t your site do too? (I repeat, don’t do that.) First, if you have the time and are using separate mobile URLs, I would recommend working towards a responsive design. Using the same URLs, even if only partially, makes things much easier.
If you’re starting from scratch, it makes sense to canonicalize and index the mobile version. However, switching canonical is very hard, and you can’t trust canonical links long term (mobile -> desktop, desktop -> mobile, etc.). You’ll need a new “link rel alternate desktop” and you’ll need to adjust all search engines. So either leave it as it is (canonical means equal anyway), or take steps towards responsive design.
FYI, by “canonicals” I mean the “link rel=canonical href=URL” element in the HTML or HTTP response header. Nothing to do with images, but technical SEO is gear, right?
When asked why sites don’t do this correctly, he responded: “I hope there are few new sites doing this, but changing the infrastructure of larger sites (like Facebook and YouTube, which I believe both use m-dot) must be a lot harder than it would be for me to post about it here.”
John added in the comments that this is something I wasn’t entirely aware of: the vary header:
Just a little anecdote, I checked with the mobile indexing team and it turns out that Google doesn’t use the “vary” HTTP headers at all to understand mobile vs desktop. They’re not needed for SEO (we’ll make that a bit clearer in our documentation). They’re purely for usability and help with HTTP caching. There’s no need to remove them; they’re just not an “SEO thing”.
Forum discussion on LinkedIn.
