XML is case sensitive, so it matters how you type your elements and attributes. With HTML you get away with mixing cases, but in XML you can't. The same goes for XHTML.
Here is an example of a HTML document:
<HTML>
<HEAD>
<TITLE>Title of Document<HEAD>
</HEAD>
<BODY BGCOLOR="#CCFFFF">
This is the document content.
</BODY>
</HTML>
...and here is the XHTML version, after applying the XML well formed rules:
<!DOCTYPE html
PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/transitional.dtd"
<html xmlns="http://www.w3.org/1999/xhtml">
<html>
<head>
<title>Title of Document</title>
</head>
<body bgcolor="#CCFFFF">
This is the document content.
</body>
</html>
The attribute values are exempt from the case sensitivity rule, as well as filenames in URLs.
