Intent to Prototype & Ship: Sec-CH-UA-WoW64 Client Hint

[Mike Taylor]

Contact emails
mike...@chromium.org, jadek...@chromium.org

Explainer
https://github.com/WICG/ua-client-hints#explainer-reducing-user-agent-granularity

Specification
https://wicg.github.io/ua-client-hints/#sec-ch-ua-wow64

Summary
The Sec-CH-UA-WoW64 Client Hint is a backwards compatible shim for sites relying on "WoW64-ness" (32-bit app running in 64-bit Windows) as they transition from the User-Agent string to UA-CH. It returns a boolean value.


Blink component
Blink>Network>ClientHints

Motivation
We would like to provide a seamless transition from User-Agent-based use cases to UA-CH. The "WoW64" bit is the last missing piece to accomplish this goal.


Initial public proposal
https://github.com/WICG/ua-client-hints/issues/217

Search tags
wow, wow64, ua-ch, uach

TAG review
https://github.com/w3ctag/design-reviews/issues/640 - still open, but issues have been addressed. I'll ping the existing issue with this addition so folks are aware, but I don't expect it to change TAG's stance of UA-CH as a feature.

TAG review status
Issues addressed

Risks
We don’t anticipate any risk here, we’re just moving this bit from one API to another as a shim.

Interoperability and Compatibility
Interop: Not all browsers expose wow64-ness in the UA string today, so sites should not treat a false value as meaningful (the spec allows for always returning false).

Compat: This hint can be thought of as a compatibility shim for sites migrating from User-Agent to UA-CH, otherwise N/A.

Gecko: Non-harmful on UA-CH, but were they to implement UA-CH I would not expect them to add this hint (as they don’t expose this bit in their UA string anymore).

WebKit: No signal. I would also not expect Apple to implement this one, were they to implmement UA-CH.

Web developers: Mixed signals (https://github.com/WICG/ua-client-hints/issues/217) We know of at least one site (Meet) that uses wow64 to prevent sending a 7MB wasm binary to users were performance is known to be lousy .

Debuggability
Nothing special needed.


Is this feature fully tested by web-platform-tests?
We'll add some.

Flag name
None.

Requires code in //chrome?
False

Estimated milestones
M100

Tracking Bug
https://bugs.chromium.org/p/chromium/issues/detail?id=1288837


Link to entry on the Chrome Platform Status
https://chromestatus.com/feature/5682026601512960

[Yoav Weiss]

Wow, LGTM1

This is a tiny addition to UA Client Hints, that will enable 1:1 retrofilling.

[Rick Byers]

Although the utility of this specific bit does seem questionable, I'm 100% in agreement that having a 1:1 mapping to UA string is a critical property and should lower both the perceived and real risk of UA reduction efforts. So emphatic LGTM2 from me.

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[Chris Harrelson]

LGTM3; I agree that the long-term benefits of this project outweigh the weirdness of this client hint.

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