Quoting the spec:
  If the cascaded value of a property is the unset keyword,
  then if it is an inherited property, this is treated as
  inherit, and if it is not, this is treated as initial.

Spec in question:
  http://dev.w3.org/csswg/css-cascade/

Also use unset in the reset-to-defaults.css we use to reset css in
reftests.