In IE the errors are like:
Syntax error: WebResource.axd Line: 1 Char: 1 Code: 0 URI: http://intranet/WebResource.axd?d=vpxv...
If I view them in the Firefox error console the errors look like:
Error: The stylesheet http://intranet/WebResource.axd?d=vpxv... was not loaded because its MIME type, "text/html", is not "text/css".
Source File: http://intranet/Pages/home.aspx Line: 0
(Note: I've truncated the URI of the WebResource.axd in the question here to fit it on the screen.)