Rocket Validator focuses on HTML and Accessibility checking, and currently it
doesn't fully support CSS checking - this is in our roadmap, but until it's ready
there are ways to check CSS in Rocket Validator as well.
These are your options:
Our HTML checker will also check CSS contained inside
<style> elements or
style attributes. So while your styles should go into linked stylesheets, you could try
including the CSS inside the HTML documents during the development/checking phase so they'll be
validated by the HTML validator.
Directly checking the stylesheets URL
Our spider doesn't pick up linked stylesheets from your HTML pages yet, but you can validate your stylesheets with Rocket Validator by submitting their URL. Just remember to disable the Accessibility checking option in this case.
Using an XML sitemap for your linked stylesheets
If your site has several linked stylesheets and all of them are inside the same domain, you could define an XML sitemap to check them at once.