Validator of page XHTML:
http://validator.w3.org/



Validateur of style sheet:

http://jigsaw.w3.org/css-Validator

V?ficateur of
bonds:
http://validator.w3.org/checklink


Validateur of feed RSS and ATOM
http://www.feedvalidator.org/


All tests and validateurs (CSS, W3C Markup Validator,
Checklinks, RDF, P3P, XML, Log, etc.)

http://www.w3.org/QA/Tools/#validators

For those which
use the browsers Firefox or Mozilla, there exists an extension which
does all much more?et, WebDevelopper

Information source: http://www.w3.org/