Intent to Ship: CSS Shadow Parts.

[Emilio Cobos Álvarez]

As of soon (whenever the patches are reviewed) I intend to turn CSS
Shadow Parts on by default on all platforms.

It has been developed behind the layout.css.shadow-parts.enabled preference.

Blink has shipped it for a while and Safari implements it in the latest
TP and is enabled by default on trunk, afaict.

We had an incomplete implementation (without part forwarding) for a
while enabled, to support our front-end code, which has been using it
quite heavily, so I'm pretty sure it's solid.

I implemented the missing bits in bug 1559076 and related.

Bug to turn on by default:
https://bugzilla.mozilla.org/show_bug.cgi?id=1559074

Standard: https://drafts.csswg.org/css-shadow-parts/

Platform coverage: All

DevTools bug: N/A, Devtools works fine with this feature already, parts
show up correctly in the inspector and their rules work fine.

web-platform-tests: We'll be passing all tests in
https://wpt.fyi/results/css/css-shadow-parts

Secure contexts: Other browsers are shipping this in non-secure
contexts, but even with that I'd be somewhat skeptic of the value of
restricting this CSS feature to secure contexts.

Is this feature enabled by default in sandboxed iframes? Yes