Nu Html Checker is now the default, for markup other than HTML5 you may use the legacy Markup Validator through custom task settings.

Select a task

Checks validity against CSS 1, CSS 2, CSS 2.1 and CSS 3 profiles

Validate by URI

Enter the URI of a document you would like checked

Advanced options
Validate by File Upload

Choose the document you would like checked

Advanced options
Validate by Direct Input

Enter the document you would like checked

Advanced options