The W3C Markup Validation Service
This validator checks the markup validity of Web documents in HTML, XHTML, SMIL, MathML, etc. If you wish to validate specific content such as RSS/Atom feeds or CSS stylesheets, MobileOK content, or to find broken links, there are other validators and tools available. As an alternative you can also try our non-DTD-based validator.
The W3C CSS Validation Service
Note: If you want to validate your CSS style sheet embedded in an (X)HTML document, you should first check that the (X)HTML you use is valid.
8/9/2010 · HTML Example: The obsolete but permitted HTML 4.01 Strict DOCTYPE with only the public identifier. <! DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN"> For the permitted DOCTYPEs that also have a system identifier, the syntax is: HTML Example: The obsolete but permitted HTML 4.01 Strict DOCTYPE with both the public and system identifiers. <!
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This document has been reviewed by W3C Members, by software developers, and by other W3C groups and interested parties, and is endorsed by the Director as a W3C Recommendation. It is a stable document and may be used as reference material or cited from another document.