In their classic **Cascading Style Sheets: Designing for the Web** (1999), Hakon Wium Lie and Bert Bos summarize guidelines:
1. Put content in its logical order !
2. Test the legibility of your documents without stylesheets !
3. Test your documents on several browsers !
4. Always set colors with numbers (hexa or RGB) !
5. Always set font sizes with ems !
6. Always specify a fallback generic font !
7. Use relative units for lenghts (ems or %) : make scalable style sheets !
8. Use floating elements instead of tables !
9. Use class attributes for the most important elements !
10. Know when to stop !
These guidelines are still valid, and not only to style-sheet !
