Servo's implementation of Sniffing in a style context

[TimvdLippe]

What is the issue with the MIME Sniffing Standard?

In servo/servo#42037 I have updated Servo's mime classifier to handle a style context. It took a bit to figure out what was the magic incantation, but eventually I ended up with something small that appears to be passing WPT:

supplied_type.clone().unwrap_or_else(|| {
    if no_sniff_flag == NoSniffFlag::On {
        mime::APPLICATION_OCTET_STREAM
    } else {
        mime::TEXT_CSS
    }
})

In other words:

1. We take the supplied type if it exists
2. If it doesn't exist and nosniff is passed, we return a dummy value
3. We fallback to text/css

Note that when an invalid Mime type is passed (text/invalid or broken), then it becomes None, hence we consider it as "doesn't exist".

Not sure what the status of this spec is and whether folks are interested in speccing the "Sniffing in a style context", but hopefully it is useful to know what Servo had to do.

[GPHemsley]

Thanks for this info!

Have you found the existing WPT tests comprehensive in this area? I was under the impression that this spec is somewhat lacking in test coverage.

Also, does your understanding of "context" align with what's on https://wiki.whatwg.org/wiki/Contexts? (See also: #97.)

[TimvdLippe]

Have you found the existing WPT tests comprehensive in this area? I was under the impression that this spec is somewhat lacking in test coverage.

The two relevant tests that I used were https://wpt.fyi/results/html/semantics/document-metadata/the-link-element/link-style-error-quirks.html?product=servo and https://wpt.fyi/results/html/semantics/document-metadata/the-link-element/link-style-error-limited-quirks.html?product=servo The latter is considered the base case, while the former has the exception. The latter is therefore confusingly named, but with it I was able to implement the logic as posted.

Also, does your understanding of "context" align with what's on https://wiki.whatwg.org/wiki/Contexts? (See also: #97.)

Tbh, that's mostly a guess. The relevant part of the spec is https://html.spec.whatwg.org/multipage/links.html#link-type-stylesheet:content-type-3 which calls https://html.spec.whatwg.org/multipage/urls-and-fetching.html#content-type which is a minimal description. Based on the context of CSS, I assumed it would be style. Preferably this would be called out explicitly like I implemented in https://github.com/TimvdLippe/servo/blob/ee86dc6d916afcb78c2022a88204e27d4809854c/components/shared/net/lib.rs#L1108-L1127 where we pass the load_context from the caller (in this case the stylesheet_loader which therefore implies style context) to the mime_classifier