Pass in Content-Encoding to resource-timing

[Jxck]

These changes are to support addition of content type attribute to Perfomance resource timing. Further details are available at
w3c/resource-timing#381

• At least two implementers are interested (and none opposed):
  • Discussed and agreed at Feb 29, 2024 W3C WebPerf call:
• Tests are written and can be reviewed and commented upon at:
  • https://chromium-review.googlesource.com/c/chromium/src/+/5369775
• Implementation bugs are filed:
  • Chromium: https://issues.chromium.org/issues/327941462
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1886107
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=271632
  • Deno (not for CORS changes): …
• MDN issue is filed:
  • New PerformanceResourceTiming.contentEncoding field mdn/content#32823
• The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

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Preview | Diff

[annevk]

I think we need to actually define how the value is obtained. "extracting header list values" shouldn't be used for new features.

And similarly to Content-Type we probably want to minimize the information that is exchanged through some sort of safelisting.

[Jxck]

@annevk Thanks for reviewing. I'm new to this area so I wanna ask you something.

we need to actually define how the value is obtained.

Do you means that this change require "extract Content Encoding" algorithm like "extract a MIME type" ?
https://fetch.spec.whatwg.org/#concept-header-extract-mime-type

we probably want to minimize the information

I think it should be seems like restricted to well-known codings like

• identity ?
• deflate
• gzip
• br
• zstd

is that right ? or is there some lists which I should refer to ?

[annevk]

Yeah, extract a MIME type is an example of modern-style header parsing. I think all things written in terms of https://fetch.spec.whatwg.org/#concept-header-list-get or https://fetch.spec.whatwg.org/#concept-header-list-get-decode-split are good examples.

There is a registry for content codings here: https://www.iana.org/assignments/http-parameters/http-parameters.xhtml#content-coding. I'm not sure if we have to acknowledge all of them however. exi and pack200-gzip don't need to be supported for instance and x-gzip can be normalized to gzip.

[annevk]

See also #716.

[yoavweiss]

We should also include br-d and zstd-d

[Jxck]

@annevk Thanks, I'll be digging them.

@yoavweiss in this case, should new encodings be added to the IANA list first ?