Also, there is currently a bug in the tests and you must have a forward slash at the end of the <link> tag just before the closing >.
If you fix the two issues above then you will get one more hint that tells you exactly what to add to the link. Again, this is actually a bug because it is not mandatory to add this particular attribute to link in a CSS file.
I believe both of these bugs are in the process of being fixed but the fixes haven’t quite made it to production yet.