CSS Guides and Reference

• CSS: Cascading Style Sheets | MDN
• CSS reference | MDN

CSS Validator

• W3C CSS Validation Service
  http://jigsaw.w3.org/css-validator/

CSS Specifications (likely not too useful)

• CSS Current Work (CSS level 3)

Historical (likely not too useful)

• CSS 2.2, April 2016
• CSS 2.1, June 2011
• CSS 2, May 1998
• CSS 1, December 1996; Revised January 1999)