Do you want to know how the CSS WG works? Fantasai has written about:csswg, An Inside View of the CSS Working Group at W3C.
box-shadowis not suppressed by
border-image. Rationale: It’s useful in many cases to have both, and the author can usually suppress it himself as needed.
background-clip: content-box. Rationale: The content box is useful for positioning, but not useful as a clipping rectangle, and having this value makes the
backgroundshorthand difficult for getting the useful case (positioning by content-box, but clipping per border-box) and easy for getting the not-useful case (clipping to content-box).
::selectionfrom Selectors Level 3. Rationale: It’s vastly underspecified, and implementations are not interoperable. We can add it back in Level 4 once we have a good definition.
Postponed discussion of margins at page breaks to Tokyo F2F.