Add note about 103 and HTTP/2

[tunetheweb]

Adds a small note clarifying 103 Early Hints recommendation to only use with HTTP/2 or later unless it is known the implementation supports this, as noted in the IETF RFC, and explains why examples are still in HTTP/1.1-style.

• At least two implementers are interested (and none opposed):
  • Already implemented in Chrome (103 - nice coincidence!) and Safari (16.6 and 17). WIP for Firefox.
• Tests are written and can be reviewed and commented upon at:
  • Clarification. Tests already written at https://github.com/web-platform-tests/wpt/tree/master/loading/early-hints
• Implementation bugs are filed:
  • Chromium: Only implemented for HTTP/2
  • Gecko: Does support over HTTP/1 but still behind a flag.
  • WebKit: Only implement for HTTP/2
  • Deno (only for timers, structured clone, base64 utils, channel messaging, module resolution, web workers, and web storage): …
  • Node.js (only for timers, structured clone, base64 utils, channel messaging, and module resolution): …
• MDN issue is filed:
  Clarification PR raised here: Add note to 103 Early Hints about recommended to use HTTP/2 mdn/content#28214

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

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/acknowledgements.html ( diff )
/semantics.html ( diff )

[domenic]

LGTM! I didn't know that this was "HTTP/1.1-style notation"; I thought it was just HTTP notation.

Do you want to add yourself to the acknowledgments section?

[tunetheweb]

LGTM! I didn't know that this was "HTTP/1.1-style notation"; I thought it was just HTTP notation.

This is HTTP/1-style:

103 Early Hint
Link: </image.png>; rel=preload as=image

200 OK
Content-Type: text/html

HTTP/2 doesn't have status name after the status code, and uses :status pseudo header so would be more like:

:status: 103
Link: </image.png>; rel=preload as=image

:status: 200
Content-Type: text/html

But the HTTP/1.1-style is so ingrained that that just looks weird to us!

But it was enough to cause confusion internally here as to "why are all the examples in HTTP/1 when no one supports that yet?"

Do you want to add yourself to the acknowledgments section?

Done!