[css-scoping-1] Dynamic changes of state and attributes referenced by :host-context rules are pretty hard to handle efficiently.

[emilio]

In particular, there's no way to look into which shadow host is affected by a host-context selector without actually going through all the descendant shadow hosts of the element where the change is detected (and that'd be slow).

Blink supports this correctly restyling the whole subtree, link below, but I think that's pretty unfortunate, because that means that every change to any class, attribute, or any other state referenced in a host-context selector needs to do very expensive work that would otherwise be unnecessarily.

Additionally, you need to store the host-context rules out of band (outside of the shadow host style), because of the same reason, which is also not great, I think. This means that stylesheet data in shadow trees is no longer self-contained.

I was looking into implementing this (and other bits of Shadow DOM / CSS scoping) in Gecko, and I'm not opposed to taking the same approach, but I'm not sure if this is intentional or not, and I'd want it to discuss it before...

• https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/css/RuleFeatureSet.cpp?l=187&rcl=b2f8374043f52c891a6927b28b00a1b4dcc02249

[emilio]

FWIW, this was discussed in the F2F, and the conclusion was that Blink will add counters and try to drop it, and that we should get feedback from other Apple people more familiar with Shadow DOM.

I guess given #3699 the second part is already there, and given it only has support from one implementation it doesn't really have a good reason to be in the spec.

[emilio]

Ah, apparently the IRC logs went to #1915 (comment) instead of here.

[emilio]

Anyhow, I'm in favor of dropping it.

[rniwa]

To repeat myself once again, Apple's WebKit team doesn't think changing the style of an element based on where it appears is a good practice to design a re-usable element in general, and this particular feature poses a significant implementation complexity & cost as pointed out by @emilio.

Support dropping this feature from the specification as we've previously requested.

[lilles]

Added separate use counters for live and snapshot profiles in Chromium M75.