On Friday 4th November, Google announced on its blog that it has started experimenting with making its index mobile-first. In fact, as more and more searches happen on mobile, Google is planning to release an important update globally, after which the mobile version of a website will be crawled and indexed as the primary source for content and ranking signals for your site.
While initially it seemed like it was planning to develop two different indexes (one for mobile and one for desktop), it's wording implies that we'll probably have just the one - but this isn't a final decision yet, as we're still months away from the launch.
While this shouldn't affect responsive sites (or dynamic serving sites offering the same content for both devices) too much, you may need to review your strategy if your mobile version is not as optimised or content-rich as your desktop one. In fact, the page speed of your mobile site will now determine the rankings of your mobile site and desktop site in the SERPs. Google is also likely to look at your title, H1 tags, structured data and other tags as well as content generated from your mobile site and use those rather than the information from your desktop site.
A few changes are also on the horizon: with desktop sites, content hidden in tabs or accordions is not weighted as highly. But with the new mobile-first index, content like this will be given full weighting if done for user experience purposes, said Google's Gary Illyes. The idea is that expandable content makes more sense on mobile and less so on desktop.
Another area of consideration is that mobile content tends to have fewer links than desktop content, despite Google's search results still being very dependent on your backlink profile. So, will the rankings be impacted? Google said it's still testing, so it isn't 100% clear at the moment.
There are other aspects which were part of the experiment but we don't know all of the details yet, and it seems it's too early to see any changes or to be sure of the final release date (we're surely months away!). That being said, the experiment is now live for a subset of users and will gradually be rolled out to more and more testers until, eventually, it replaces the desktop index fully.