If you have ever maintained a web site you know that pure html is difficult to mantain and impossibile to change without doing it all again from scratch (even if you use css).
XML once again is the solution to all of your (web) problems. By using XML and a couple of XSLT sheets you can easily skin your site whilst keeping the content in one separate, safe, upgradeable place.
If you want a tool that will make you started quick go have a look at Apache Forrest.
[shameless plug] My web site was made with forrest: www.unicolet.org