3.4.3 CSS
In the mx-element% method descriptions, “CSS” refers to
  the Cascading Style Sheets specification.  A CSS length is string
  consisting of a decimal integer number followed by one of the units
  px (pixels), em (font height), ex
  (height of an “x”), in (inches), cm
  (centimeters), mm (millimeters), pc (picas), or
  pt (points).  A CSS percentage is a string consisting of a
  decimal real number followed by %.  When using
  -native methods, CSS lengths and percentages are
  given as strings.  For use by non-native methods, the
  css-percentage and css-length structures have been
  defined.