W3C
www.w3.org
W3C; the World Wide Web Consortium. This is like the backbone of web design. If you are new to web design you may find this site heavy going, in fact you may not find it of much use. but as you learn about the various coding languages used in website design there is no doubt you will come across this site at some point. It holds all the specifications for the most popular web design languages; html, CSS, Xhtml and many more.
If there is one mistake I made when I started designing websites, it was not taking enough notice of this site and the foundations of coding. If you visit this site every now and then as you are learning to write websites, you will not regret it. It will give you a great understanding of coding, the structure of the whole thing and it will help you to understand how comprehensive the languages actually are and how much you can really create with html, css etc.