I'm glad to see someone so dedicated to publish an accessibility-first UI kit! I've reviewed the accessibility for 21 different UI libraries [1] and only one was fully accessible (according to my criteria). I'd like to add your library to my list, however, the color contrast of your button component is not sufficient (3.98 - it should be 4.5 for AA).

Another small issue I've found: the "npm install reakit" textarea is focusable but doesn't have a focus ring (your code examples all do). It's not as important, as this only affects your documentation page, but I'd either add a focus ring or disable the focus altogether (as the field is not editable).

[1] https://darekkay.com/blog/accessible-ui-frameworks/