http.headers.Trailer - reading trailers not supported by Fetch API

[lidel]

What information was incorrect, unhelpful, or incomplete?

Trailer support table suggests "trailers" are supported by all browsers:

THIS IS NOT TRUE: trailers are not supported at all.

The current state is harmful: having Trailer header marked as green in that table suggests this is a usable feature, but reading trailers is not supported by Fetch API in any browser.

Readers expect to be able to act on trailers advertised in that header, and they waste hours trying to figure out what they do wrong, only to find that... no major browser engine supports reading trailers via Fetch API (from JS):

• Firefox: not supported – https://bugzilla.mozilla.org/show_bug.cgi?id=1339096
• Chromium: closed as Wont Fix – https://bugs.chromium.org/p/chromium/issues/detail?id=691599
• Webkit: https://bugs.webkit.org/show_bug.cgi?id=168232

What did you expect to see?

• Remove confusing "Full support", or at least rename that row to "Trailer (HTTP header to indicate trailers to expect)" or similar
• Add new row that shows broken status for "Reading announced trailer headers with Fetch API" in all browsers

Did you test this? If so, how?

• Cannot use [SameObject] for trailers whatwg/fetch#473
• Access to the HTTP trailer whatwg/fetch#34
• Error handling for streaming API handlers in the browser, has it ever worked? ipfs/js-ipfs#2519
• Trailers removed from Fetch in Remove trailer support whatwg/fetch#979
• Wider context on how Google let this happen: https://carlmastrangelo.com/blog/why-does-grpc-insist-on-trailers

MDN page report details

• Query: http.headers.Trailer
• MDN URL: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Trailer
• Report started: 2022-01-28T17:40:31.410Z

[kmannislands]

+1 for this. Fairly misleading support table, definitely needs a heavy caveat!

Noticed that trailers are documented but red here in the Response doc but that seems rather subtle and there's no note anywhere or mention in the support table: https://developer.mozilla.org/en-US/docs/Web/API/Response