[css-shadow-parts] Let ::theme() penetrate closed shadows

[tabatkins]

In #2368 (comment), Ryosuke comments:

There should be no difference between closed and open shadow trees for ::theme. ::theme should always penetrate shadow boundaries. Otherwise, it defeats the primary use case of ::theme.

I would think that letting ::theme() penetrate closed shadows defeats the primary purpose of having a closed shadow in the first place. I'm happy to go either way, tho.

[Jamesernator]

You have a note in proposed spec

It’s important to note that ::part() and ::theme() offer absolutely zero new theoretical power.

However this isn't true, you already say in the following statement that you can avoid overstyling. This happens because ::part/::theme can't accidentally inherit styles whereas custom properties can.

With that in mind, if ::theme pierces shadow roots in the same way that custom variables do, is there any point in having ::theme over just implementing @apply, as it seems to me that any ::theme(some-name) would be identical to @apply(--some-name) under this shadow-piercing scheme.

[tabatkins]

No, @apply is still substantially different and suffers from several additional problems beyond what ::theme does. I outlined these in https://www.xanthir.com/b4o00.

[Jamesernator]

I see, ::theme definitely seems like a better replacement for the functionality of @apply.

[equinusocio]

@Jamesernator ...and @apply is officially deprecated and will never be supported :)