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Generally speaking HTML tags are case insensitive. Attribute values on the other hand may well be case sensitive, for example the TARGET attribute values are case sensitive... _top, _blank, _parent, and _self. Also, filenames (such as page.html) are usually case sensitive (depending on the server), and JavaScript, although technically not part of HTML, is VERY case sensitive.
Thanks to Joe Barta for amendments to this FAQ.
The following was submitted by Dan Souza
Although tags are not case sensitive in the current version of HTML, the next generation of HTML which is XHTML (extensible hypertext markup language) does strictly require that all tags be in lower case.