Flash
It's possible to build libogg, libvorbis, libtheora etc to ActionScript3 (Flash) bytecode using the CrossBridge compiler.
A development version of ogv.js used to include such a Flash build, however development was stopped on it due to lack of maintenance on CrossBridge and general limitations in the Flash platform. If there's interest -- and maintenance on CrossBridge picks up -- it could be restored, benefiting users of older browsers (IE 9 and below, Safari 5 and below).