Google has started experimenting with a "mobile-first" index that primarily ranks sites based on their phone-friendly pages.
Today, most people are searching on Google using a mobile device. However, Google's current ranking systems still typically look at the desktop version of a page’s content to evaluate its relevance to the user. This can cause issues when the mobile page has less content than the desktop page because Google's algorithms are not evaluating the actual page that is seen by a mobile searcher.
To make the results more useful, Google has begun experiments to make its index mobile-first. Although the search index will continue to be a single index of websites and apps, Google's algorithms will eventually primarily use the mobile version of a site’s content to rank pages from that site, to understand structured data, and to show snippets from those pages in the results. Of course, while Google's index will be built from mobile documents, Google is going to continue to offer search services for all users, whether they come from mobile or desktop devices.
Google said it would continue to experiment over the coming months on a small scale and will ramp up this change later on.
Websites that currently have a responsive site or a dynamic serving site where the primary content and markup is equivalent across mobile and desktop, shouldn’t have to change anything.
But webmasters that have a site configuration where the primary content and markup is different across mobile and desktop, should consider making some changes to their sites.
Google added that sites do not have to make changes to their canonical links; the search giant will continue to use these links as guides to serve the appropriate results to a user searching on desktop or mobile.