As more and more searches happen on mobile devices, Google wants its index and results to represent the majority of their users — who are mobile searchers.
Although Google wants you to have a mobile version, if you don’t have one, it will crawl your desktop version instead.
If you have two versions of your website, desktop and mobile, and the mobile version has less content, Google will probably just see the mobile version with less content.
This is why Google recommends you go with a responsive approach — the content is the same on a page-by-page basis from your desktop to your mobile site.
Google said they have already begun testing this mobile-first index to some users. Google won’t give us a date for this index to roll out because it is dependent upon how the testing goes.
Google has previously said that content that’s not deemed mobile-friendly will not rank as well. That remains the case with this new index.
As Google grows confidence in the mobile-first index, eventually that will be the only index used. Or if the new index isn’t deemed useful, Google could go back to a desktop-first index. It has, after all, called the mobile-first index an “experiment.”
Google said in their blog post, “Our search index will continue to be a single index of websites and apps, our algorithms will eventually primarily use the mobile version of a site’s content to rank pages from that site.”