Units: pixels, ems, display resolution

This is a proposal to add 7 units related to graphics, imaging, typography.

We could consider adding a new category for these, say “graphics”; otherwise some of them do not fit reasonably into any existing category.

Per TC meeting 2019-05-22:

    • Use singular for the internal key name, e.g. “dot-per-inch”

    • Put all of these in a new category “graphics”

Some reference material: