This Wednesday Yandex announced [in Russian] that from now on, the font size of texts on mobile web pages is yet another factor added to a new ranking algorithm Vladivostok. To remind us, Vladivostok was rolled out a month ago and takes into account mobile-friendliness of a page in its ranking. Checking texts for mobile optimization is already added to the testing tool in the new version of Yandex.Webmaster:
Now, if a font size of a mobile page is at least 12px, Yandex considers it mobile-friendly and hence, gives it a better ranking in the mobile search results. If the text readability of web pages on mobile devices is not optimized according to Yandex, their ranking in the mobile SERP will be lowered.
And, let’s keep in mind – even if Yandex has already recognized pages as mobile-friendly before, they will be re-tested and checked for all the ranking factors again, this time including the font size.