HTML? No. The practice of lower case came about for XHTML, which being in a XML namespace means tags should be lower case. For consistency sake and so everyone is on the same page, and for ease of switching between types, it's generally accepted that anything with upper case tags in it predates HTML 4. Kind of like naming your files ".htm" instead of ".html" reeks of "what is this, 1994?" It doesn't ACTUALLY matter, but that's the last time such things were accepted practice.
HTML is NOT case sensitive as the others replied. Just have something in mind. Depending on the underlying hosting OS (or actually the filesystem) you may experience some problems with file names. In windows there is no difference whenever you use upper or lowercase files, but in Unix/Linux it matters. If you host your html files under Windows index.html and iNdEx.html will open exactly the same file, but in Linux those are two different files. So always stick on lowercase filenames for compatibility.
http://lmgtfy.com/?q=is+HTML+case+sensitive%3F&l=1 Try to google it it's easier for all of us, in this case you mostly
No html is not case sensitive . But i think its good practice to use same case upper or lower in one tag. For example <P></P> or <p></p>.