SV, CV and CSS2-like inheritance [was: Re[2]: [cgmo-webcgm] Style properties]

[Lofton Henderson <lofton@rockynet.com>]

At 02:58 PM 5/17/2005 -0400, Benoit Bezaire wrote:
>[...]
>LH> A.) If the Computed Value of a style property involves "convert to
>LH> absolute",
>LH> B.) and if we inherit the Computed Value,
>LH> then .... it creates problems for CSS2-like inheritance of style
>LH> properties.
>What are these A) and B)? I'm not sure I'm following your line of
>thoughts here?

Okay, A and B are two factors or conditions that, taken together, make it 
difficult to apply the CSS2 model, unmodified, to our style properties 
inheritance.  We are talking about inheritance down a tree of APSs.

A.) CSS2 says that convert-to-absolute happens at the Computed Value stage.

[Stroke-weight is a property of an APS.  If convert-to-absolute for 
stroke-weight means that you must convert % to NVDC, this cannot be done at 
the APS level.  So the CV of stroke-weight is ill-defined at the APS level, 
if the CV requires convert-to-absolute.]

B.) CSS2 says that Computed Values are inherited.  (CSS3 allows each 
property to define which type of value is inherited.)

Condition A and condition B taken together makes it problematic to apply 
the CSS2 inheritance model exactly to our style properties inheritance.

If we relax condition B, i.e., if we say that SV is inherited down APS 
branches (for style properties) instead of CV, as allowed by CSS3, then 
that inheritance model works.  (But it leaves CV pretty much unused in the 
model, and doesn't solve the problem of uncomputable CV at the APS level.)

If we relax condition A -- i.e., convert-to-absolute happens after CV, 
e.g., at the Used Value stage -- then CV is the same as SV (except for 
resolution of the values 'inherit', which we don't allow anyway for style 
properties.)  And inheritance down APS branches works, CSS2-like.

-Lofton.