In HTML, tags and attributes are not case sensitive. This
means that <BODY>,
<body>, <Body>, and even
<bOdY> refer to the same thing; you can
have your opening tag be <body> and your
closing tag </BODY>, and it all works
fine.
XML is case sensitive:
<Body> is considered a different tag from
<BODY>, <body>, or
<bOdY>.
Because of this, a standard for XHTML tags had to be set -
and the most likely choices were "all uppercase" (such as
<BODY>), or "all lowercase" (such as
<body>). In the end, lowercase won out,
although it really could have gone either way; a semi-
arbitrary choice had to be made.
So - element and attribute names must be in lower case. So
do attribute values listed in the DTD, as in <p
align="left"> (Transitional DTD
example).