xml-coding.el is a spot of code to select an Emacs CodingSystem? from the
<?xml> tag at the start of any XML document.
This is designed for Emacs 21. Emacs 22 already has this feature builtin, so nothing is done there.
Most XML files are UTF-8, so you may find it enough to just add to
.xml or whatever filename extension you’re working with (see UnicodeEncoding for more on UTF-8).