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The current region styling syntax uses ::region() to match the proposed ::distributed() in Shadow DOM. Since this change, the Shadow DOM folks decided to change ::distributed to use a space separator after the pseudo-element instead of parentheses. Once this change makes its way into the Shadow DOM spec, the Regions spec should change to follow.
https://dvcs.w3.org/hg/csswg/rev/d6c2c35066c7