Why is site code validation important ?
* mostly consistent display for your sites with many browsers (which display the one and same - unvalid - page very differently) and making your site more readable for different user agents
* helping exposing and repairing barriers to accessibility
* encouraging compliance with existing accessibility guidelines
* making websites accessible to all individuals, including those with disabilities
* demonstrating professional coding skills
For HTML / XHTML / WML / XML / RSS / ATOM / Google Sitemap:
http://www.validome.org/Validome allows Web Publishers to check their syntax with a reliable, high-speed validation service, in accordance to current official Standards. Valid code is very helpful, in order to avoid problems with different browsers and releases. A simple visual check of your site does not conform any more to modern webdesign and generally accepted technical requirements.
For CSS:
http://jigsaw.w3.org/css-validator/W3C CSS Validation Service; a free service that checks Cascading Style Sheets (CSS) in (X)HTML documents or standalone for conformance to W3C recommendations.
Accessibility:
http://webxact.watchfire.com/WebXACT is a free online service that lets you test single pages of web content for quality, accessibility, and privacy issues.