How can a user of libcss distinguish between these two situations: a)
property was defined using a default value in CSS and b) property was
undefined in CSS for the node. Those are different because in a latter
case the value of that property should be taken from a node attribute
(in my case of svg styling). As far as I can tell, for a root node, the
property is always initialised using the default value, so when
composing a style I can't distinguish between a and b.