Now playing midi in html is supported with javascript and windows media player/quicktime browser plugins. I'm wondering if there are any news about html5 is going to support playing midi with audio tag in the future ?
If you want to play midi files, you can see how they did it here: http://mudcu.be/midi-js/. The example page makes use of MIDI.js. Note that it even uses soundfonts, all via scripting. Pretty neat.
If you're more interested in controlling midi devices, or having midi devices control your HTML5 page: the W3C guys are working on that. See the draft api description here