Below I listed resources which both developers and designers might find useful. These include links to sites of web standards advocates, web design gurus, etc.

Web Standards

World Wide Web Consortium

The Importance of Web Standards

This is an article published at sitepoint.com

Building ASP.NET 2.0 Web Sites Using Web Standards

ASP.NET and XHTML

List of valid DTDs

CSS

Every link from the 'Developer Reads' on your right goes here as well.

CSS2 Specification

A List Apart

If you don't have this one bookmarked yet you're missing out a lot!

Eric Meyer's web site

CSS Hacks

Another outstanding collection of CSS hacks

CSS Page Layout Templates

Listamatic

A showcase of what you can do with lists

Little Boxes

An excellent collection of page layouts

CSS Crib Sheet By Dave Shea

The Layout Reservoir