Google's ranking criteria are constantly changing, depending on user preferences.
A significant change in the ranking algorithm is the prioritization of mobile versions of sites over desktop versions. Your website needs to be mobile friendly in order to find a place in the top search results.
The Google Mobile-Friendly Testing Tool serves you to measure the ease of reading and navigation, as well as to make suggestions for improvement. It looks for issues like flash, horizontal scrolling, and inappropriate fonts that may seem harmless but can put you way ahead of the competition or far behind.