Currently 99% of the world-wide websites are incompatible with old browsers, other web devices (mobiles, palmtop, pda´s, etc) and inaccessible for the visual and physical deficients. The W3C (World Web Consortium) created rules, standards and recomendations so that the Web could be developed with quality and accessibility for everybody. To have a W3C validated website is not just a status, its a necessity and a global tendency. Among hundreds of advantages, here are some of them:
The website becomes from 30% to 70% faster
The files are reduced to 1/3 of the original size
The maintenance becomes faster e easier
Any device can successfully access the website (WebTVs, Smartphones e Palmtops, etc)
Accessible for visual and physical deficients
The website is correctly viewed in any browser (old or new versions) or OS
It becomes really optimized to search engines through several techniques.