In addition to what @valex has said, I want to recap a couple of things with the given answers for this question.
I think the general answer is: it depends on the programming language that you are using.
C Sharp
Microsoft has written some guidelines where it seems that HtmlButton is the right way to name a class for this cases.
Javascript
Javascript has some global variables with acronyms and it uses them all in upper case (but funnily, not always consistently) here are some examples:
encodeURIComponent
XMLHttpRequest
toJSON
toISOString
