When you're just starting with css, this is great as an offline reference. Download it now and then you can use it easily while you're experimenting:
http://www.htmlhelp.com/distribution/css.zip (windows)
http://www.htmlhelp.com/reference/css/
http://www.thenoodleincident.com/tutorials/box_lesson/boxes.html
http://www.tantek.com/CSS/Examples/boxmodelhack.html
http://www.nypl.org/styleguide/
http://www.mako4css.com/
http://css.nu/