[css-shadow-parts-1] vendor prefixed pseudo elements support

[liamdebeasi]

As per the spec, the ::part() pseudo-element can take additional pseudo-elements after it such as ::before (source: https://drafts.csswg.org/css-shadow-parts/#part).

Does this extend to vendor-prefixed pseudo-elements as well? The current behavior in Chrome and the latest Safari Tech Preview (96 as of writing) is that prefixed pseudo elements do not work with shadow parts.

Example: https://codepen.io/liamdebeasi/pen/jOOpQKa

I'm wondering if this is just a browser bug, or maybe a part of the spec that just needs clarification. Thanks!

[Offroaders123]

I recently ran into this issue too, and I was wondering if we know whether support for it will be added at some point? Thanks!

[tabatkins]

Undefined, basically by definition - they're vendor-prefixed because they're not standardized, so the CSSWG doesn't define how they act. This should be raised against the browser vendor to decide.

I can see arguments either way, fwiw:

• On the pro side, this seems self-evidently useful.
• On the con side, this exposes internal details of how the part is implemented. This is precisely why sub-parts have to be explicitly forwarded by the component for them to be visible in the outer page. (::before/etc are present on all elements, so it doesn't expose anything, beyond just whether or not the component is also using the pseudo.)
• Back on the pro side, components can forward sub-parts into their own exposed parts, surfacing useful bits when needed. But there's no syntax for forwarding built-in pseudos, so forbidding it by default would also forbid it at all times.

Overall, I suspect the morally correct answer is to lean towards not exposing them, but I suspect the practically correct answer is to expose them.