CSS HTML Validator supports a variety of web standards and languages, including, but not limited to the below:

HTML 5.3 (preliminary)

HTML 5.2

HTML 5.1


CSS4 (preliminary)


HTML 4.01

CSS 2.1

WCAG 1.0 and WCAG 2.0 accessibility checking


HTML Microdata


MathML (new v12.00)

JavaScript (supports JavaScript linting using a 3rd party linting program like JSLint)

PHP (supports PHP syntax checking using 3rd party PHP software)

Section 508 Accessibility (For the US Government)

CLF 2.0 (Canada Common Look and Feel 2.0)

In addition, please keep in mind:

At the time of this writing, some standards are currently a work-in-progress, like HTML 5.3 and CSS4.

CSS HTML Validator is not meant to provide total and complete support of the above standards and languages. Some parts of the above specifications may not be supported or the checking may not go 100% by the specification.

CSS HTML Validator is designed to be a 'practical' validator, based on the standards but also based on what works 'in the real world'. We believe this makes CSS HTML Validator more practical & useful to web developers.

NOTE: CSS HTML Validator is not a DTD validator. However, the standard and higher editions include nsgmls, a DTD validator.

See also: Program Limitations