Canvas mostly works across browsers but there are still variances and browser-specific edge cases you have to detect. Cross-browser audio is an absolute trainwreck, and APIs like gamepad, fullscreen, etc. are all very finicky as well and will require careful effort if you want to use them in each browser. Plus, each JavaScript runtime has things it likes/doesn't like so to get playable performance for a serious game in all the browsers you'll have to spend a while figuring out how to tune your JS for all of them (or writing browser-specific versions). This is made worse by the fact that some of the major browsers don't have usable profiling tools...

But sure, if all you want to do is draw some lines and bitmaps on the screen, no problem.