Réf. : Re: [css3-fonts] Nested 'bolder' and 'lighter' question

I don't think it's stated explicitly anywhere, but I it seems natural to 
me that bolder and lighter should be symmetrical.
In other words, if you apply bolder n times and lighter n times you get 
back to the original weight. 

Some of the proposals involving clamping seem to break this principle, 
making it impossible to "keep track of desired boldness."

LB