The changes rolling out today are intended to make it easier for mobile search users to land on pages that are much easier to view on smaller screens. From Google:
Google notes that while the change to rankings will seek out mobile-friendly pages, it won't be the only factor in rankings. If a site provides high-quality content, but isn't mobile-friendly, it can still rank highly in mobile search results.
As noted above, the update only applies to searches on mobile devices, and is rolling out today to all search results in all languages. If you're a web developer curious to see if your site makes the cut, Google has provided a tool to test just that.
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