Like in SMIL, display of certain elements can be submitted
to constraints ( same module as SMIL )
Attributes :
xml:lang
system-required = list of functions
Defines the minimal functionnalities which should be implemented
For instance
SVGLang : all the functionnalities described in the specification
SVGStatic : a subset
SVGDOMStatic : + DOM interfaces corresponding to SVGStatic
SVGDOMDynamic , ....
