well you'd need to be an approved developer by nintendo I think, which means you'd need to fill out a developer applicaion https://developer.nintendo.com/register/create
and I think you also need to buy a devkit but I am not sure about it, as all people in the program are under non disclosure agreement so nothing filters (I am under no NDA since I am not in this program).
However do know that the HTML5 on wii U using the nintendo web framework simply does not work with webGL, and as a friendly reminder, WebGL isn't just for effects, it ensures the game runs fast, so I would not even bother, all the things I heard is that it is clunky, and not great perfs wise due to that.
More infos should be discussed on the developers forums of nintendo I guess, but that requires a developper account.