mobile and desktop devices, because although html is supposed to already be cross platform, in theory, when it comes to games you have to make some assumptions about whether a user is using a mouse or a touch screen or a keyboard, specific screen sizes, testing on obscure browsers running linux and old versions of gecko and things start to break
Disclaimer: I really don't know anything about doing games in html/js. The question is honest.
Disclaimer 2: I do remember "write once, run anywhere". It was only mostly right back then.