I don't read HTML all the time, and I don't expect it to be easy to read when outside of a browser.

Markdown though, that's my slightly fancy readme. It has just enough structure that I can easily read and understand it on the command line.

I can easily grep it without need for an advanced parsing engine.

If you need something with font and style weight then go for HTML.

But if I see README.html with a project, it's going to put me off wanting to contribute. I do not want to learn that project's style guide for how to add some new parameters to a table or remember that they want to specify italics in one of a dozen different ways.