Proposal:

During parsing of a css declaration block, shorthands (like margin and all) expands only to the set of longhand (grouped by mapping logic: so either logical or physical) required to describe their value, and by default resort to use physical properties if both sets are possible. Note that they can only expand if they do not contain var() substitutions.

During serialization, each logical property group is considered, and shorthand values can be emitted instead of the longhands only if the all the longhands of the same mapping logic are present and consecutive (ignoring all properties that are not in the group). Otherwise, the shorthand is not used when serializing the properties in the group.

During serialization, a specific rule should be added in case all longhand properties of one mapping logic are defined, and are followed by all longhand properties of the other mapping logic (this would result in the shorthand being emitted twice, in which case the rule would discard its first appearance)

ex:

margin-top: 0px; margin-left: 0px; margin-bottom: 0px; margin-right: 0px;
margin-inline-start: 10px; margin-block-start: 10px; margin-inline-end: 10px; margin-inline-end: 10px

should serialize as margin: logical 10px (if supported) or just margin-inline-start: 10px; margin-block-start: 10px; margin-inline-end: 10px; margin-inline-end: 10px (otherwise), ignoring the set of longhand properties of physical mapping logic whose all values are to be ignored in the end.

When removing a shorthand property, all the longhand associated with that shorthand should be removed, regardless of their mapping kind.

[css-logical-1] Interaction of shorthands and logical properties #3030

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