Here are some excellent notes from the W3C:

- http://www.w3.org/Provider/Style/

Web pages should always include a DOCTYPE_ header with markup that conforms to the selected standard. Checking pages with a validation service is an excellent way to quickly verify your markup:

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

See also: `Automatic HTML Validation`_

.. _DOCTYPE: http://validator.w3.org/docs/help#faq-doctype