There are some quirks and issues with HTML5 at the moment, but it's really on a fantastic trajectory.
When we first released C2, only desktop Chrome and desktop Firefox supported 2D canvas (no WebGL yet) with software rendering only, so you'd be lucky to get 30 FPS even on a high-end gaming machine. No mobile devices could even start up the games, and the latest version of IE was IE8 which didn't know what the <canvas> tag was.
Within 2 years, it's a substantially different picture. All desktop browsers have at least good support, and many mobile devices have good support (especially Chrome and Firefox on Android). Hardware acceleration is pretty much a given (last hold-out is the Android stock browser, which is well on the way to being replaced by Chrome for Android as the default).
I expect the next 2 years to bring the same level of improvement. IE 11 will support WebGL. Firefox will soon enable their Web Audio API implementation (which fixes the looping issue). WebGL support has doubled in the past year and according to Mozilla's stats is now supported on about two-thirds of all machines (including Windows XP, but up-to-date drivers are harder to come by on XP). The next version of Chrome for Android will support WebGL and it already supports the Web Audio API. And that's just the next couple of months.
I know people need to work on projects now, and these are real issues. But things are fast improving, and hopefully any long-term projects will see the issues solved before they go to launch.