Why using CSS style sheets ? Why not other configuration methods like a XML file ?

In specifying graphic style for elements drawn by the graph viewer we felt that a CSS-like syntax was the more appropriate since:

a lot of people already know CSS well and have a good understanding of its concepts ;
a rule based syntax was quite appropriate to apply style to general things like “all nodes” and particular things like “the node with identifier ‘A’”;
there was already a well documented whole dictionary of styling attributes in CSS that we could re use.
separating the graph structure from its appearance will be quite useful.
Using style classes and dynamic size and color we can quickly change the appearance of a node or edge just by changing one attribute.
It is often more efficient since you do not have to apply style to each element individually. Furthermore at runtime the styles are compiled in a graphically efficient way that allows to apply them and switch them quickly for elements of the graph.