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http://www.westciv.com/style_master/academy/css_tutorial/index.html
— This may be the best, by far, to learn CSS — it’s a handbook of sorts, that walks you through CSS development. This is where I got my start with CSS.

http://webreview.com/pub/guides/style/mastergrid.html
— This is a grid of CSS support by browser put together by WebReview. Good when you have to figure out why a feature isn’t rendering right, or if you need to develop for a specific browser and don’t know what features aren’t supported right.

http://www.blooberry.com/indexdot/css/index.html
— Once you have a handle on the format and syntax of CSS, this is, BY FAR, the best reference source. It comes from Brian Wilson, who does Index Dot HTML as well; it’s a clikable dictionary of CSS terms and what they mean, and also contains browser peculiarities for each. Great, can’t say enough about it.

http://msdn.microsoft.com/workshop/author/default.asp
— This is Microsoft’s reference source. I use it a lot since I have to develop for IE for most of my projects; other than that, though, it is a great place to see what features are out there, and to see code examples of each.

http://www.w3.org/Style/CSS/
— This is the official W3C resource for CSS. It has their reference sources, and a good what’s new list of things that have to do with CSS (new browsers, editors, and the like).

http://css.nu/
— This is a community-run website (the community being the people from comp.infosystems.www.authoring.stylesheets) that serves as a clearinghouse of CSS information. Very thorough.

comp.infosystems.www.authoring.stylesheets
— The newsgroup. Said to have less meaningless chatter than a lot of Usenet newsgroups these days.