There's also https://www.radix-ui.com/ which is excellent.

In what way is Headless UI 'headless'?

It doesn't come with any styling OOTB. The idea is that you're just building a frontend (all the markup, styling etc) around some provided state management

`react-table` is headless. I am forced to create my own html structure and am in control of the markup from the start. `headless-ui` provides those components which doesn't give me full control over markup, does it?

It gives you complete control over your markup and styling. Headless UI is designed to handle everything except what the components look like.

<Component as="some html tag"> is not full control :)

Styleless would be a more apt name.

This came before it I think, and even before that I know ReachUI and Reakit