[selectors] Pseudo-class to indicate when a slot has content

[mozilla-apprentice]

A resolution was made for csswg-drafts/#6867.

[selectors] Pseudo-class to indicate when a slot has content

• RESOLVED: `:has-slotted` should match when the fallback content is not being displayed

Discussion.

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To file a bug automatically for these resolutions, add the bug label to the issue.

If no bug is needed, the issue can be closed.

[dholbert]

@emilio I wasn't following the discussion on this one, but I think you were involved -- could you either file a bug [via the bot or directly] if one is merited, or close this if we don't need to do anything?