There were quite a few games built with HTML5 in the 10K Apart contest. The grand prize winner's submission was a game built using just canvas and JavaScript, measuring only 6.9KB.
HTML5 is indeed making it easy to develop games for standard web browsers while also providing a way for developers to reach mobiles and tablets with a single code base.
The Google sponsored HTML5 Game Jam has also produced some nifty games. Check the winning entries and also look out for Mozilla's Game On competition