Unlike SGML, element names and attribute values are case-sensitive in XML (i.e. no 'case folding' takes place during processing). For this reason, all attribute values should be specified as lower-case by default except where case is used to distinguish one attribute value from another.
For example, the listgroup type attribute uses the HTML list attributes of a, A, i and I to distinguish different styles of list numbering (i.e. lower case alpha, upper case alpha, lower case roman, upper case roman). In this example, the case of the letters is significant. However, as of 2000-11-03, this is the only example of attributes being distinguished by case in the DTD.
Filenames of XML files and any files associated with XML files must be named in lower case and identified in the XML file using lower case characters.