SVG 1.2: Something Missing?

Hello,
 
We’re currently evaluating RCC to implement into an authoring tool.
We had a good progress but came to a big issue. Say someone writes a new
widget with rcc
and the elementDef Element, how can the end-user know which attributes
this new widget accepts?
E.g. I create a progressbar with a “progress” and a “max” value, how can
I know that these two attributes
are available and which type (SVGLength) they have? Surely I could look
into the documentation from the author
if there’s one but how can an authoring tool know that? There must be a
REQUIRED tag in the elementDef Element 
where you need to setup all the available properties to let a custom
widget become valid or any other solution
that goes into that direction. Could you please help me out how that’ll
work or should work? We’re currently hanging on this problem.
 
thanks,
 
mfG / Sincerely
Alexander Adam
EvolGrafiX -  <http://www.evolgrafix.com> http://www.evolgrafix.com
SVGCafé - http://www.svg-cafe.com