W3C links
- W3C XHTML Validator
- Validate your XHTML in transitional, strict, frameset, and more. Debugging extreme or minute errors can really make a difference in how your site works. It can really make the site look cleaner.
- W3C CSS Validator
- Validate your CSS to have cleaner code. If something's is not working out well with your code it could be a syntax error and this will find it for you.
- W3 Schools
- Learn about a myriad of XHTML, CSS, Javascript, PHP, and more. You can also get some quick examples and take quizzes to test your knowledge. Try it out.
- W3.org
- The guys who make the official standards for the web. If they don't agree with it, then good luck. Have you ever read the standards on their websites? It's very hardcore and detailed but make sure you bookmark this website because they have everything.