I wanted to make a game once using HTML5/JavaScript, but when I learned you can't even properly gaplessly loop an audio file without using non-standard extensions, I quit and moved on to Flash.
Then I realized that Flash has a similar issue and if you want to properly loop audio, you have to have Flash Professional (I was using FlashDevelop). So instead I wrote a program that can generate SWF files that contained gaplessly looping MP3s that I could import into FlashDevelop, instead of buying Flash Professional. It was a fun experiment, but at the same time it means I didn't get to work on my actual game for quite some time. The good thing is that at least I could work around the issue in Flash, unlike with HTML5/JavaScript (well, I could have used non-standard JavaScript APIs to gaplessly loop audio, but I wanted the game to work in all major browsers with your "standard" setup, and I'm pretty sure that would've been more work to "get it right" with all the different APIs).
Moral of the story: HTML5/JavaScript are pretty limited (especially if you just stick to what's standardized, and like others have said, support for the latest standard is quite fragmented, which sucks even more). Working in Flash can give you some additional capabilities/speed that straight HTML5/JavaScript might be lacking, but if you don't get Flash Professional you'll still hit some annoying obstacles.