Picking a font size for the web used to be really easy. 16 points and you’re done. You could feel confident that your content would be decently readable on PC or Mac, Netscape or IE. Man, have times changed.
Typography
What's the Best Font Size For the web? Well, It Depends… — Imarc
W3Schools' statistics may not be relevant to your web site. Different sites attract different audiences. Some web sites attract developers using professional hardware, while other sites attract hobbyists using older computers. Anyway, data collected from W3Schools' log-files over many years clearly shows the long term trends.