Clarify WebTransport BFCache interaction

[nidhijaju]

Based on w3c/webtransport#500 (comment), this PR explicitly calls the "context cleanup steps" in the WebTransport spec from within the HTML spec's "unloading document cleanup steps".

cc @rakina

• At least two implementers are interested (and none opposed):
  • Chrome
  • Firefox
• Tests are written and can be reviewed and commented upon at:
  • Add BFCache WPT for pages with WebSocket/WebTransport/WebRTC web-platform-tests/wpt#40117
• Implementation bugs are filed:
  • Chromium: https://bugs.chromium.org/p/chromium/issues/detail?id=1440026
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1829996
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=255973
• MDN issue is filed: …

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

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/acknowledgements.html ( diff )
/document-lifecycle.html ( diff )
/infrastructure.html ( diff )
/references.html ( diff )

[rakina]

Thanks a lot for the quick work here! Non-owner LGTM :)

[domenic]

Generally looks good with minor editorial issues! I think WebTransport has other implementers involved too, right, so more than just Chrome can be said to be interested in this?

Web platform tests would also be good, ideally ones that show that WebTransport shutdown happens after WebSocket shutdown but before EventSource shutdown, since that's where you placed this.

[nidhijaju]

@jesup or @jan-ivar, can we say Mozilla is also interested in this?

[jan-ivar]

Yes Mozilla is interested.

[nidhijaju]

I've updated the PR description with this information as well, but we have implementer interest from Mozilla now, and there are some WPTs at web-platform-tests/wpt#40117. Does this look good to you, @domenic?

[domenic]

Looks great, thank you!