Both Chrome and Safari allow propagation of `::selection` styles into the Shadow DOM. I think Firefox should allow it too because a selection is generally not considered part of a element, but part of the browser's UI and therefor needs no encapsulation.
Bug 2016387 Comment 1 Edit History
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Both Chrome and Safari allow propagation of `::selection` styles into the Shadow DOM. I think Firefox should allow it too because a selection is generally not considered to be a part of an element, but part of the browser's UI and therefor needs no encapsulation.