M arkup L anguage. There's no matter if you use lower or upper case letters and tags. Even IE 2.0 doesn't require those tags.
Tags are commands to do something.
Always remember, HTML is a language of tags and it is completely based upon tags. However you can also declare JavaScripts in BODY tag. This is important so anyone can know who's that web site is and what is it about etc. Hello my name is Whiz . You will notice that you page is completely black.