CSS content has errors
This means that the URL in question is a CSS file that contains one or more syntax errors.
Why is this important?
If a CSS file contains syntax errors, this causes the declaration or selector to be invalid, and when this happens the browser just ignores it completely and moves on to the next one it finds. Depending on how critical the invalid style is, this could result in the layout going badly wrong.
What does the Hint check?
This Hint will trigger for any internal or external CSS URL which contains syntax errors.
Examples that trigger this Hint:
The Hint would trigger for any style sheet that contains syntax errors, such as:
How do you resolve this issue?
If Sitebulb flags CSS errors, you can dig into the issues further using a CSS validation tool such as W3 Validator.