Tidy is a syntax utility that reads HTML, XHTML and XML and writes clean markup. The utility works pretty well. Tidy is included with Tiger by default and I could see potential uses of tidy creating valid XML and XHTML. You can even use tidy to convert from HTML to XHTML.
Tidy can be installed on earlier versions of OS X or any UNIX OS for that matter:
http://tidy.sourceforge.net/cooper:~ scotbuff$ tidy index.php
line 1 column 1 - Warning: specified input encoding (iso-8859-1) does not match actual input encoding (utf-8)
Info: Doctype given is "-//W3C//DTD XHTML 1.0 Strict//EN"
Info: Document content looks like XHTML 1.0 Strict
No warnings or errors were found.Review the man page for tidy and use tidy's help screen -
tidy -h
