To learn how to create webpages:

http://www.alistapart.com/

Jeffrey Zeldmans website. Probably one of the biggest promoters of the W3C web standards.

http://960.gs/

Practical fluid CSS layout templates. Just study the source code to learn.

http://www.csszengarden.com/

Old but still relevant showcase gallery based upon a single piece of text, resulting in hundreds of sometimes very different designs.

http://www.w3.org/Style/CSS/Overview.en.html

CSS overview from W3C.

http://jigsaw.w3.org/css-validator/

Validate the code you have written and learn.