[css-box-4] Longhand values of margin-trim allow more combinations than shorthand values #7884

With the change of #6643 we can now have different combinations of block-* and inline-* values for the margin-trim property.

The current syntax is:

none | block | inline | [ inline-start || inline-end || block-start || block-end ]

This syntax allows combinations of *-start and *-end values that for both block and inline directions. Currently, those cannot be represented by the shorthand values block and inline.

To allow to express those cases in a more condense way, I suggest to allow to combine the longhand and shorthand values.

The syntax would then be:

none |
[ block || inline ] |
[ block || inline-start || inline-end ] |
[ inline || block-start || block-end ] |
[ inline-start || inline-end || block-start || block-end ]

Sebastian