There are currently several specs defining different ways for referencing (ID) selectors:

• CSS UI 4: <id>
• CSS Images 4: element(<id-selector>)
• Scroll Animations 1: selector(<id-selector>)

To improve the consistency between them, the syntax for referencing a selector should be merged.

Personally, I prefer the selector() function of Scroll Animations 1 because its name clearly outlines that it takes a selector and it is extensible as may allow other selectors then ID selectors in the future.

See also the related discussion in #5166. And depending on the outcome of #1981, CSS Images 4 might be changed to not have a specific selector function anymore.

Sebastian