The first step on the way to templates will be to clean up the existing code in-place, making it valid XHTML as much as possible.
I've put some tools in CVS that will help us work on this.
The second change is a modification to lib/setup.php that will allow the above-mentioned validation services to have access to your site. By default it will allow them in as guest, but if you create a new user with the username 'w3cvalidator' then they will be logged in as that user. Obviously you can give that user any privileges you like to access any pages.
So, if you have access to CVS, please find a page that needs looking at, fix any bodgy old HTML tags in there and check it in. Please work on one file at a time to minimise conflicts, since there will be several people (myself included) climbing all over this thing. Also, please avoid the temptation to add new features along the way, this is just a clean up.
I will write separately about some guidelines for the actual tags to use.