I am quite sure that speedtest.net is correct because
HTML5 "is a markup language for structuring and presenting content for the World Wide Web..."
"SpeedOf.Me is a HTML5 Internet speed test," which means one is measuring actual World Wide Web page speeds. Some of Youtube videos (without advertising) are using HTML5 but it must be enabled: See: »www.youtube.com/html5/ And when I come across a HTML5 Video, they will load extremely fast.
Here's a video, but HTML5 must be enabled on your browser