Most "native" apps, are native in the sense that they use some some UI toolkit that renders using the platform specific and typically hardware accelerated ui infrastructure.
With webgl and wasm, those same toolkits can now work in a browser.
As for accessibility, that is indeed a challenge. You would need to use some ui toolkit that supports accessibility features. Those of course do exist, browsers don't have a monopoly on accessibility.
A related challenge is the integrating that with the browser properly so that the native accessibility facilities in your OS can do whatever it needs to do (screen reading, enhancing contrast, etc.). This is not a solved problem. But it sounds like it could be a solvable problem that would require some changes to wasm and webgl.