[CSS21] font-size percentages on root

The 'font-size' property states in 15.7:

   # Percentages:  	refer to parent element's font size

This doesn't define what happens on the root element.

The definition for percentages in 4.3.3 states:

   # When a percentage value is set for a property of the root element
   # and the percentage is defined as referring to the inherited value
   # of some property, the resultant value is the percentage times the
   # initial value of that property.

I think it's pretty clear that these two statements are supposed to
link up, but they don't. So I propose changing "parent element's
font size" to "inherited font-size".

~fantasai