Sites typically won’t get punished for having different content on mobile and desktop
Because Google has switched to mobile-first indexing, this means that Google very rarely crawls the desktop version of a site. Therefore, punishment for differences between the versions is unlikely, as any differences between mobile and desktop probably won’t even be detected. Googlebot does crawl the alternate version of a site from time to time, but this is primarily to confirm the connection between the two, rather than to compare the content.
Google Will Only Use Content From Mobile Pages if Mobile First
Google will only use the mobile version of content when a site has been moved to mobile first. If your mobile pages have less structured data or images than your desktop pages, Google will not use the content from your desktop site. Google will continue to crawl your desktop pages about 20% of the time to make sure they are not missing any new pages.
Google Recommends a Single Site for Mobile First Indexing
It’s OK to have a separate mobile website for mobile first indexing, provided it’s correctly linked from the desktop version with a mobile rel alternate and a canonical pointing to the desktop page, but Google recommends having a single URL for each piece of content with responsive design or dynamic serving in the future.
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.