John Mitchell (Mr. Snapsis.com) maker of the Snapsis menu and PageBlaster has a site with the front page xhtml strict. Note: The rest of the site is non-compliant.
He accomplished this using PageBlaster ($49). I have used this tool to make my site
www.AgingSafely.com mostly xhtml transitional. Beside speedng up you DNN installation greatly by caching whole pages PageBlaster is a
regular expression replacement engine.
You can write a regex rule to replace < img with < img alt="" or what ever is required.
PageBlaster is a Swiss Army Knife with twelve razor blades. In the right hand you can do fine surgery. In the wrong hand you may become one-handed.
PB comes with the rules that John uses for his site. Not all worked for me. It was a good starting point, which is what he intended it to be.
It was relativly easy to "lower tags, close tags, fix hr & br, nowrap, fix t css and script type statements, fix img statements and deal with some other minor hacks.
I haven't been able to force "quoting attributes", l"ower all attributes" etc.
John's rules for these broke the world on my system. I have a lot of regex experience from the Unix world and Perl and I have yet to get these rules so that it doesn't kill DNN somewhere when I enable it on the whole portal.
Going in and fixing the DNN source code is the other option. Some of it is easy and others places are very hard. Unlike Unix open source projects making DNN isn't a simple "make clean, make all, make install" after you have edited a few lines of code. If you are asp-person then this may be the way to go.
Good luck,
/DaveS