I remember poking around at the Wii U browser. Nintendo had examples of fetching the current state of buttons, analog sticks, and the touch screen to monitor for input.

While cool on paper, there wasn't a preventDefault() solution. So you could make a simple game where a sprite could move around and respond to "A," but if you press B, the browser would try to go Back a page. As the article mentions, the shoulder buttons activated a Gyro-based scroll mode (which wasn't great). "B" would go Back a page, Y would close/open the "curtain" on the TV, X would open the URL bar (thus showing the software keyboard and taking over all inputs), and Start/Select also did something, although I've since forgotten what.

So, although all button inputs were present, almost all of them also did something on the browser level, so nothing exciting ever came of it.