We expected this was coming and even heard about it over a year ago, but this is the first time Google posted details about the mobile-first index on their blog.
Most Google searches are mobile, but Google’s index is desktop
Google explained that it sees more mobile searches than desktop searches daily. But when Google evaluates a page’s ranking in Google, it currently looks at the desktop version of the site — an issue we pointed out over a year ago. To fix this, Google will look at the content, links, and structured data of the mobile version of your site if one is available.
To make our results more useful, we’ve begun experiments to make our index mobile-first. Although 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, to understand structured data, and to show snippets from those pages in our results. Of course, while our index will be built from mobile documents, we’re going to continue to build a great search experience for all users, whether they come from mobile or desktop devices.
With this change, Google will primarily index mobile content and use that to decide how to rank its results, regardless of whether you’re on desktop or mobile. There will no longer be any “mobile-friendly” adjustment done just for mobile users. Effectively, if you’re not mobile-friendly, that will have an impact even on how you appear to desktop searchers.