Michael Deal made a brilliant music theory learning app for Google Chrome using base64 soundfonts. In fact he is not really using the soundfonts itself, but rendered .ogg files that are loaded as base64 files into the html5
Deprecated, please use Chris Wilson’s WebMIDIAPI Shim I made a test version of the midibridge that allows you to playback midi files. This version uses the File API for loading the midi file and a html5 slider for controlling the song position, so it currently only runs in Chrome: