Those puzzled by the terms "HTML" and "CSS" may want to read the article What is HTML, CSS, Java Script, PHP and Perl? My primer on accessibility, How to Improve the Accessibility of Your Website may be useful too.
You can use the W3 Consortium's HTML validator to check your web documents either by giving it your web page's URL or uploading your document directly to the validator.
The executables may already be downloadable by the time you read this, but if they are not, you can still find older versions at the original HTML Tidy Library Project page with binaries for Windows, Mac OS, Atari, Amiga, Be OS, AIX, Linux, Unix Ware, HP-UX, MSDOS, Solaris, OS/2, Free BSD, RISC OS, Mi NT (Atari) OS, etc. This is an online version of the HTML Tidy validator that you can use to clean up your web pages if you're too lazy to download your own copy.
(To get your own copy, see elsewhere on this page - listed under "HTML Tidy".) The Web Page Purifier allows you to check your HTML pages for compliance ("purity") with various HTML standards (eg HTML 4.0 Transitional, HTML 4.0 Strict, Web TV 1.1, HTML 3.2, etc).
Be sure to let us know if you have any feature suggestions using the Feedback link on the left side.
This validator checks the markup validity of Web documents in HTML, XHTML, SMIL, Math ML, etc.
Note: if you are not sure what HTML and CSS mean, please read What are HTML, CSS, Java Script, PHP and Perl? For those unfamiliar with the term, "validating" a page is just a jargon-filled way of referring to the use of a computer program to check that a web page is free of errors.
In particular, an HTML validator checks to make sure the HTML code on your web page complies with the standards set by the W3 Consortium, the organisation ("organization" in US English) that issues the HTML standards.
The W3 Consortium has its own online validator which you can use for free.
Registration only takes a minute and is of course perfectly free.
We are also planning on adding more features to user accounts in the future, such as more advanced comparison options.
To validate a CSS file, enter your site into the field below and click on validate.
Note: This validator will only validate CSS, to validate HTML code please use this html validator.