Google has announced that it made “significant updates” to its developers documentation on mobile-first indexing. You can see the new docs over here and here is the cached version that is old (updated in October 2019).

Here is Google’s announcement:

I am currently on vacation, so I don’t have time to dig through a diff on the changes between the two but thankfully we have Twitter. Aymen Loukil posted the differences he spotted on Twitter. Here are the updates he found:

Focus on having “same content” on Mobile VS Desktop
Same meta robots
No interaction based lazyload
Resources must be crawlable
Same structured data on both versions
URLs in Mobile structured data must point out to mobile ones
Same meta tags [title, description]
High Quality images
Persistant images URLs
Same alt text for mobile/Desktop images
Use supported video formats
Don’t put the video in the bottom of the page

Here are best practices for separate URLs (m dot):
Mobile version shouldn’t give an error
No fragment URLs for Mobile
No one for many mobile URL
Verify both versions on Search Console
Same robots.txt file
Correct Canonical and Hreflang implementation

Aymen added more, but here is where he started posting this:

Thanks Aymen!

Forum discussion at Twitter.