When performing a single-URL or full-site CSS validation, you can choose to ignore CSS validation errors from chosen domains.
This is useful if you link to third-party CSS resources through a common content delivery network; you can ignore CSS validation issues for resources over which you have no control.
For each result collected from the CSS validator, the URL of the CSS resource to which the result relates is checked. If the domain in the URL is on the list of domains you have chosen to ignore the result will then be ignored.
This process was recently failing for one specific URL being tested.
The URL of the stylesheet was very similar to the above illustration. Notice anything wrong with the markup in the illustration?
Bonus points if you notice that the Content-Type meta element is present twice but, no, that's not the answer I'm looking for.
"Someone is developing on Windows?", you guess. No, that's not it either.
The answer I'm looking for here is that the CSS resource uses a
URL to reference a file on the local file system.
Such a URL won't work on the Internet, which will likely adversely affect the given page's appearance, but that's irrelevant to us as the markup is valid.
The problem is that a local
file:// URL contains no domain
If, like us, you're checking the domain in a URL against a list of domains that you want to ignore, you're might well run into problems if a given URL doesn't contain a domain and if you'd not expected this to happen.
And that's exactly what happened and it's that exact case that I've just fixed.
Feel free to now use valid but somewhat silly local URLs to reference stylesheets. It's a bad idea if you want your page to work on the Internet but at least it'll no longer break our CSS validation process.