[css3-sizing] fill-available in block direction and infinite size

Given the definition of available size:
  available size 
  The space into which a box is laid out, typically either a measurement of its containing block or an infinite size.
and the definition of fill-available (extent):
  fill-available extent 
  Roughly, the extent a box would take if it filled its available extent. 
I’m asking myself the question: what should be the size of a BODY whose height is set to fill-available? Infinite? How can we deal with this?

I propose to modify the definition of available size to state something like:
  available size 
  The space into which a box is laid out, typically either a measurement of its containing block or of the viewport height if it is infinitely sized.
As such, in the example I propose, BODY would have a 100vh height.

What do you think of it?
François