Talking about the slider specifically, I'm of course not advocating for going out and rewriting all sliders using shaders.

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.