The most common resolution people use for HTML5 games would probably be 640x960 (Portrait).
The reason for portrait is, that if you want your game playable on a mobile browser, it is less intrusive and there is no danger of browser bars minimizing your game (as in landscape).
It is also debatable, that more people use their phones in Portrait mode and prefer casual games that way.
If you're not designing for the browser, and instead want to release an App, then it is up to you whether you choose Portrait or Landscape, and will largely depend on the game you are making.
There are very successful games in both orientations.
The third option is to make your game responsive, and playable in both orientations. This however opens up a whole new can of worms, as you have to adjust your menus and game assets dynamically, so they best fit the screen in the given orientation.
Hope this helps :)