HTML tags are not case sensitive:
<CENTER> <Center> and
<center> all work the same way. However, XHTML, the next generation of web markup language, will require lower-case tags, so we recommend using lowercase now to ease conversion in the future.
There are some special and extended HTML characters (not tags) that are case sensitive. These are discussed in the Intermediate HTML and web publishing tutorial.
URLs (web addresses) are case sensitive, so when you are creating links in your HTML code, be sure to type in the correct letter case for the web address you are linking to.