Google Determines Mobile Usability Based on The Ability to Render the Page
Google May Still Crawl Parts of a Site With Desktop Crawler
Even with the shift to mobile-first indexing, Google may still crawl parts of a site with the desktop crawler. John explained that this will not impact the site as long as things are working well on mobile.
Canonical Tags Should Remain The Same Between Desktop & Mobile Sites Even With Mobile First Indexing
If you have a separate m. version of your site, the canonical should remain pointing to the desktop version, despite mobile-first indexing. This is because Google will use the canonical tag to understand which of the pages belong together. Internally, Google will pick the mobile version as canonical.
Google Uses the Viewport Tag to Understand the Mobile-friendliness of a Page
The viewport tag is used for scaling a page for devices, and Google will use this tag to understand whether the page is mobile-friendly or not. Google won’t automatically categorise a page as a mobile version if it has a viewport tag.
Mobile-first Readiness Classifer Will Review Content On Both Mobile & Desktop Sites
With mobile-first indexing, Google will only index what it is able to crawl on the mobile version of the site, so if there is important content on the desktop version which is not available on the mobile version, this will not be indexed. The readiness classifier will be used to recognise if the content is similar enough on both, before switching to mobile-first indexing.
A Readiness Classifier & Algorithm is Used to Determine Mobile-first IndexingA Readiness Classifier & Algorithm is Used to Determine Mobile-first Indexing
A readiness classifier is used to determine whether or not pages are ready for mobile-first indexing and looks at elements like internal linking, anchor text and content differences. However, if the algorithm determines that the majority of pages are ready then the site will be moved, even if some pages have content inconsistencies. This could impact rankings.
Mobile-first Indexing Isn’t a Ranking Factor or Quality Signal
There’s no need to push a site to be moved to mobile-first indexing. This is only a technical change in the way pages are crawled and indexed, and being in the mobile-first index doesn’t come with any quality signals or ranking boosts.
Incorrectly Using the Vary User Agent HTTP Header Won’t Stop Site Being Moved to Mobile-first
Using the Vary: User-agent HTTP header incorrectly isn’t ideal, but Googlebot will still be able to find content on the page to index and won’t be impacted much.
Having Different CMS’ & Different Configurations Across a Large Site can Delay Mobile-first Indexing
If you have a large site that uses a combination of different CMS’ and front-end and back-end configurations, this can cause Google’s classifiers to determine that your site shouldn’t be switched over to mobile-first indexing. For large sites, it can be normal for a small percentage of URLs to be moved across in batches.