The existing values for "display" have long conflated "inside" and "outside" display models, so it can't really be separated now. But at one point there was a

The existing values for "display" have long conflated "inside" and "outside" display models, so it can't really be separated now. But at one point there was a proposal to add two new properties, "display-inside" and "display-outside". And "display" was then redefined as a shorthand which combined both. This have been removed (https://www.w3.org/TR/css-display-3/#changes-wd https://www.w3.org/TR/css-display-3/#changes-wd) with this justification: Removed display-inside, display-outside, and display-extras longhands, in favor of just making display multi-value. (This was done to impose constraints on what can be combined. Future levels of this specification may relax some or all of those restrictions if they become unnecessary or unwanted.) It seems this is to restrict combinations with "display:none" and "display:content" and the various "display:table-*".