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

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Mike Taylor

Jan 25, 2022, 9:11:32 AMJan 25
to blink-dev, Ali Beyad,

Contact emails,



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

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

Search tags
wow, wow64, ua-ch, uach

TAG review - 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

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 ( 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 .

Nothing special needed.

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

Flag name

Requires code in //chrome?

Estimated milestones

Tracking Bug

Link to entry on the Chrome Platform Status

Yoav Weiss

Jan 26, 2022, 10:23:01 AMJan 26
to blink-dev, Mike Taylor, Ali Beyad,
Wow, LGTM1

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

Rick Byers

Jan 26, 2022, 11:07:40 AMJan 26
to Yoav Weiss, blink-dev, Mike Taylor, Ali Beyad,
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

Jan 26, 2022, 11:16:31 AMJan 26
to Rick Byers, Yoav Weiss, blink-dev, Mike Taylor, Ali Beyad,
LGTM3; I agree that the long-term benefits of this project outweigh the weirdness of this client hint.

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