A demonstration on the use of CSS.
Click on any of the styles available: HTML page remains the same, only the style sheet changes.
This shows you the full power of CSS.
http://www.w3schools.com/ Literally "the school of the W3C.
Many classes to learn to use CSS, DOM, etc.
Contains examples of very practical and very basic.
Here are some rules to follow to make a site that is clear:
http://www.webstyleguide.com/ (This is a book available entirely online.)
It will give you tips and tricks to improve usability, clarity and readability of your site.
You can get a glimpse of differences in reporting between the various web browsers via the screenshots:
See what your site elsewhere
browsershots.org / recent/
http://www.validator.w3.org/ The W3C Markup Validation Service. This site will allow you to submit a URL or send a web page to check if the web standards are adhered ...
You can also find this automatic analysis in the "web developer" of the Firefox browser (see the Validation tab once installed):www.chrispederick.com/work/firefox/webdeveloper/