Stylesheet loads ignore responses with MIME types that don't match text/css

[jdm]

This means that any page relying on a file:// URL, or any webserver like the built-in python one that checks for common extensions like .css to decide what mime type to return will cause unexpected behaviour when loaded in servo vs. Firefox.

[SimonSapin]

Relevant specs:

• https://html.spec.whatwg.org/#link-type-stylesheet for <link rel=stylesheet>
• https://drafts.csswg.org/css-cascade/#content-type for @import
• https://html.spec.whatwg.org/multipage/urls-and-fetching.html#content-type
• https://mimesniff.spec.whatwg.org/

Quirks mode is relevant.

Where https://mimesniff.spec.whatwg.org/#supplied-mime-type-detection-algorithm says to use the Content-Type header for HTTP(S), for file: it says:

If the resource is retrieved directly from the file system, set supplied-type to the MIME type provided by the file system.

I’m not sure what this is supposed to mean. I’m not aware of any filesystem such as NTFS, ext4 etc encoding MIME type information. Is this saying to take a guess based on a table of "well-known" file name extensions, similar to https://docs.python.org/3/library/mimetypes.html#mimetypes.guess_type ? Probably not, as the table would hopefully be specified in that case.