The 3DS used a relatively deficient browser engine, with much less support for web standards; it didn't support anything a non-trivial game might need.

A browser based on current WebKit or Blink, with full hardware-accelerated support for WebGL, a fast JavaScript JIT, WebAssembly, fullscreen, joysticks, and other such standards would make a compelling platform.

Well, the point stands: browser and integrated pad is not enough for third party browser games to emerge. Nintendo would also have to provide relevant APIs, some of which aren't commonly used by regular websites (e.g. WebGL).