Add User Agent Client Hints

[iwasa-kosui]

Because we web application developer should try to provide apps without UA if Client Hints are implemented, It's important for we that how mozilla think about User Agent Client Hints.

https://mozilla.github.io/standards-positions/
Is it correct that you mozilla feel User Agent Client Hints and HTTP Client Hints are just not-harmful but do not want implement this now?

Please add that mozilla will not support UA-CH.

[chrisdavidmills]

Eventually we want to provide an update on platform-status where we show status for technologies that are not implemented yet, or only available in an experimental form. But we are not there yet. Hopefully we can spend some time on this near the end of the year.