We've been working for some time now on getting Slash to utilize CSS and also updating it's old crufty HTML while we're at it. We've moved Slash to HTML Strict 4.01. Slashcode.com and use.perl.org are currently running on this code. You can take a look at the markup, or log in and take a look around.
If you'd like to see what Slashdot might look like you can activate the Slashdot stylesheet on Slashcode.com in Firefox by choosing View > Page Style > Slashdot. I'm sure you can do the same thing with other browsers but you're on your own for the specifics of how to do so.