Not by design: It is possible to modify a wordpress theme for better xhtml output without modifying the theme system and the codebase. This keeps updates easy and your blog save.
As already written, I’m using a popular Blog Software right now. Next to havin’ the newz on the front, it’s about linkin’ (as it was with any page ever before as well). So this shouldn’t be something complicated, right? Well for most people it might be alright to webdesign their site, and when finished push their design around to fit with the software…
For me, this is not the case. In such scenarios it comes to advanced wordpress theming to full fill all needs. Checkout the source to gain knowledge. It’s all about links and less tagsoup then the “classic” or “standard” (or even “kuberik”) tagsoup. Keep it small and niffy. If you’re not able to, no problem: For posers, there is a sneaky post about PHP and Tidy available out there you can hack into your weblogs source (a full article in german).