[css3-values] Which font to use for the ex unit?

Hi,

css3-values defines the ex unit as "Equal to the font's x-height." (and 
then goes into details of what the x-height is) but does not say which 
font to use. This can be ambiguous eg. when font-family contains 
multiple valid family names.

For comparison, the ch unit is based on "the '0' glyph found in the font 
used to render it." which is well-defined, based on the font matching 
algorithm.

-- 
Simon Sapin