Re: [w3ctag/design-reviews] User Preference Media Features Client Hints Headers (#632)

@plinss, @torgo and myself had one more extended review of this entire proposal during our Madripoor vf2f. 

First of all, thank you for bearing with us while we've been working on this review. Unlike the UACH base feature which we support, this proposal has problems we can't endorse as a general mechanism of the Web platform.

As you mentioned - `...this is not aimed at the average website.`. Not all Web platform features are aimed or used by all websites. However, Web features must be predictable and possible for use by anyone, safely. 

The base problem of this proposal is the exposure of **user state** rather than **client capabilities** of user preference media features. This allows user state handling on the server side which can lead to race conditions and undesired error state and user experience. One that doesn't exist today with media features which are evaluated and handled on the client side. This is not a safe pattern and should be avoided.

During our call we discussed multi stakeholder interest. You mentioned of few other large Web properties that would be interested and willing to use the feature for similar benefits to those of `google.com`. That isn't surprising. They employ enough engineers and will be able to take the hit of implementation and support. What we don't see is evidence of other browser vendors interested to support the feature. 

We don't see evidence or use cases of user preference media features other than 'prefers-color-scheme'. If this is the only use case, with most benefit to your scenario, consider exposing a hint of such client capability, i.e. "does this device support auto detection of light or dark mode"

We understand the feature is already shipping in Chrome, but we can't endorse adding this as a general mechanism to the Web platform.

Thank you again for bringing the feature to us for review. If there is a future proposal that resolves our concerns, please don't hesitate to open another review.

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