See also :, :
Note that unlike all of our existing length units, pixels and ems are not a fixed size (e.g.the size of an em depends on the point size of a font, and pixels depend on display device resolution; they may have different sizes along different axes). This may need some discussion.
: requested units for dot and pixel density, or resolution: dots/inch, dots/cm, pixels/inch, pixels/cm. Note that pixels and dots may not be square.