Useful links to check W3C Quality Assurance and XML Schema

Check W3C Quality Assurance and XML Schema

Contains links to W3C Quality Assurance checker, W3C and WDG (X)HTML validators and XML Schema validator checker.

W3C Quality Assurance

The W3C Markup Validation Service

http://validator.w3.org/

The W3C CSS Validation Service

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

W3C Unicorn
the Web's Universal Conformance Checker - ALPHA Test Version

http://qa-dev.w3.org/unicorn/

W3C mobileOK Checker [Beta]
Make your Web site mobile-friendly!

http://validator.w3.org/mobile/

Other validators

WDG HTML validator, validate the entire site

http://www.htmlhelp.com/tools/validator/

XML Schema Validator,
validate XML documents such as XHTML against the appropriate schemas (only available in english).

http://schneegans.de/sv/