YoYo Games employee Andrew McCluskey has posted online the first publicly playable game created using the HTML5 version of GameMaker.
R5Maddening is a version of Remaddening created using an early build of GameMaker HTML5, which is currently in closed alpha.
Update: A HTML5 version of Jesse Venbrux’s They Need to be Fed is also available to play at YoYoGames.com
It would be interesting to know what levels of performance you experience when playing (or trying to play!) the games.