(cffi:defcenum svg-unit-t
  :user
  :em
  :ex
  :px
  :in
  :cm
  :mm
  :pt
  :pc
  :percent)  

:user

User unit, a value in the current coordinate system. If used in the root element for the initial coordinate systems it corresponds to pixels.

:em

The size of the element's font.

:ex

The x-height of the element’s font.

:px

Pixels (1px = 1/96th of 1in).

:in

Inches (1in = 2.54cm = 96px).

:cm

Centimeters (1cm = 96px/2.54).

:mm

Millimeters (1mm = 1/10th of 1cm).

:pt

Points (1pt = 1/72th of 1in).

:pc

Picas (1pc = 1/6th of 1in).

:percent

Percent, a value that is some fraction of another reference value.