Comment/question about HTML validator response...

Hello:

I used both the CSS and HTML validators today, and I have a question about
the wording and meaning of an apparently "boilerplate" statement that the
CSS validator issues. My CSSs tested without errors or warnings, but I am
wondering about the statement that was displayed every time:

No error or warning found
To work as intended, your CSS style sheet needs a correct document parse
tree. This means you should use valid HTML.

I also tested my HTML and got no errors or warnings. Can you please clarify
what the statement means?

Thank you.

Burtin Hart