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:

• https://www.w3.org/Style/Examples/007/units.en.html

• https://en.wikipedia.org/wiki/Pixel

• https://en.wikipedia.org/wiki/Display_resolution

• https://en.wikipedia.org/wiki/Dots_per_inch

• https://en.wikipedia.org/wiki/Pixel_density

• https://en.wikipedia.org/wiki/Em_(typography)