As detailed here among other places, the only valid characters in a html/css class name is a-z, A-Z, 0-9, hyphen and underscore, and the first character should be a letter. But in practice, what characters are in fact supported by most browsers? More specifically, I wonder what browsers properly understands a slash (/) in a class name, and what browsers support class names starting with a number.

I'm primarily interested in getting an answer for html rather than xhtml, in case there is a difference.

Thank you.