CSS and HTML already have several decades of work on accessibility and cross platform support. It's far from trivial to recreate this.
For things like hero pages, I can see the benefit. But for basic UI like sliders, are you reinventing the wheel here?
This is just an example of a place where you can use a shader to create a cool, interactive visual effect.
On the example site I've implemented the slider as a regular HTML slider element, and using a canvas to replace its visual appearance. Doing something like this, I don't believe there's anything in the way of getting all the accessibility benefits of a regular slider element, though I could be wrong, I'm no expert by any means.
However, from what I can see, it probably is accessible, as it is still usable without a mouse, which is an easy first test.
(Though, in Safari, neither slider test has any kind of slider, so there are other big problems.)