Why are Web Standards Important?
Here’s an explanation of web standards from The Web Standards Project:
Complying with web standards can give your web pages greater visibility in web searches. The structural information present in compliant documents makes it easy for search engines to access and evaluate the information in those documents, and they get indexed more accurately.
Accessibility is an important idea behind many web standards. Not only does this mean allowing the web to be used by people with disabilities, but also allowing web pages to be understood by people using browsers other than the usual ones—including voice browsers that read web pages aloud to people with sight impairments, Braille browsers, hand-held browsers, teletext displays, and other unusual output devices.
You can read more about developing with web standards at 456 Berea Street.
(Thanks to Sandbox, a standards compliant WordPress theme, for the way to this excellent explanation)