[css-sizing] Why is fill-available not definite?

Hi there!

According to https://www.w3.org/TR/css3-sizing/#definite-size:
definite sizeA size that can be determined without measuring content; that
is, a <length>, a size of the *initial containing block*, or a
<percentage> that
is resolved against a definite
<https://www.w3.org/TR/css3-sizing/#definite-size> size.
That does not include the intrinsic sizing keywords. If nothing else,
shouldn't fill-available be definite?

Also, the part about percentages is not correct because children of an
abspos element can have resolved percentages no matter how the parent's
size is computed (
https://www.w3.org/TR/CSS2/visudet.html#the-height-property), which this
section does not mention. Is this not a normative part of the spec?

-Christian