You can check here to see if you browser supports HTML5 .
I use Opera beta 27 . It will do all but one . The MSE & H.264 has a red box with an exclamation mark.
So far I haven't had a video that wouldn't play .Of course a lot still play on Adobe Flash. Since that has been changed I will see.
You can also set HTML5 player as the default here or set Adobe Flash.
At least for now. If you clean your cookies like I do you have to do this again.
Now that HTML5 will be the default I won't need to very often if at all.