This is part 1 of 2 about buttons. In part 2 we’ll focus on button states, WCAG Compliance (and a bit about ADA as well) and a couple more minor things.
Out of the top of my head it’s contrast + size + clarity of copy. Then on top of that you can add sound feedback / alt actions and more.
In general as far as accessibility is concerned we think about it in a broader perspective — making it accessible for everyone (and I mean “really” accessible) in many cases solves the problem for everyone.
Readability and clarity is top priority here (and everywhere with UI’s)