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Original versions of HTML were case-insensitive and, in fact, very forgiving. This means all of the following examples would be considered the same by the browser:
That said, HTML4/XHTML is case-sensitive and requires all tags to be lowercase. Of the three preceding examples, the browser might properly interpret only the first.
To make it really confusing, HTML5 returns to being case-insensitive. Given the differences between the various versions of HTML currently in use, I recommend using all-lowercase tags.