[css-gcpm] Some notes on interoperability between AntennaHouse and PrinceXML

As I've been writing tests for GCPM, I'm noticing some differences in how
PrinceXML (9.0r5) and AntennaHouse (6.2 MR1) implement these features.
Before we get to the next stage with the spec, resolving some of these
differences would be very helpful.

[1] footnote area. The spec and AntennaHouse use @footnote; Prince uses
@footnotes.

[2] content() function. AntennaHouse is fine with the default content(),
but doesn't handle the explicit version content(text).

[3] first-except is not supported by Prince.

[4] target-text: The spec and AntennaHouse use target-text; Prince uses
target-content.

[5] start keyword on string function: Prince is replacing the start value
with the entry value in all circumstances. AntennaHouse appears to be using
the value of the first assignment on page one of a document, even when the
element is not the first element on the page.

[6] content(before) and content(after) are not supported by Prince.

[7] string-set applied to an element with display: none exposes a bug in
Prince.

I believe in all these cases there are no differences between GCPM and CSS
Books.

Kind regards,

Dave