Then we need APIs that would allow GUI frameworks that render onto <canvas> to support accessibility. Such APIs have existed on desktop platforms for decades now, and popular frameworks like Qt use them, even when they draw all the widgets themselves.
By the way, Qt-on-wasm-on-canvas is already a thing: http://blog.qt.io/blog/2018/05/22/qt-for-webassembly/
Native toolkits also support accessibility.
And while accessibility is a big issue, I must sadly acknowledge that I am yet to have a project require compliance and validate its implementation, as such it never gets done.
And while it's technically possible to say, just compile QT to wasm and give it a CANVAS tag as a dumb drawing surface, you've just broken accessibility, and that's a deal breaker.