The previous commits refactored the `ShadowCss` emulator to support
desirable use-cases of `:host-context()`, but it dropped support
for passing a comma separated list of selectors to the `:host-context()` .
This commit rectifies that omission, despite the use-case not being
valid according to the ShadowDOM spec, to ensure backward compatibility
with the previous implementation.
PR Close#40494
645c2ef973