Error Empty or ruleless stylesheet
Why the validator reports an error when it finds no CSS at all, or CSS text with no actual rules in it.
The validator reports empty-css when no CSS text was provided at all, and no-rules-found when there's text but it doesn't contain anything the parser recognizes as a rule or at-rule.
Common causes
For a URL check, this usually means the fetched resource wasn't actually a stylesheet — an HTML error page returned instead of the expected .css file, a redirect that wasn't followed correctly, or a URL that pointed at the wrong file. For pasted input, it typically means the CSS didn't get copied in full, or what was pasted was plain text/comments with no real rules.
Why it matters
This check exists mainly to catch a broken input before you spend time interpreting an otherwise-empty results page as "no issues found" — an empty stylesheet trivially has zero errors, which is a misleading way to read a validation report. If you expected real output and got this, the fix is almost always about what went into the validator, not about the CSS itself.
Frequently asked questions
Why does an empty input count as an error instead of just showing no issues?
Because "no CSS to check" and "CSS that was checked and had zero problems" are very different results, and reporting the first one as a clean pass would be misleading. Flagging it makes the actual situation explicit.
I pasted CSS and still got this — why?
Check that the paste included the actual rules and not just comments or partial text, and that no invisible characters or encoding issues came along with the copy. For a URL check, confirm the URL points directly at a .css file rather than an HTML page that merely references one.