Use CSS defaults for symbolizers

[aaime]

Right now colors and sizes default to null, however SLD provides reasonable defaults, e.g., when setting a stroke width to 2 it would make sense to see a black line, not an empty map because the color has not been assigned.

Each symbolizer has basic, meaningful defaults in SLD, CSS should use those.

[dwins]

Right now the idea of a 'key' property is baked into the CSS compiler - every type of symbolizer has a single property that triggers it - if there's no 'stroke' property then there are no line symbolizers, etc. This ties in pretty tightly with how multi-pass effects are encoded. Are you proposing that I change the key property for line symbolizers, or that we change the system for encoding multiple symbolizers? If the latter, I'd like to hear a little more about the scheme you think should be used.

[aaime]

Isn't it possible to have it trigger when any of the properties in the symbolizer are used? I mean, if I'm saying I want a dash array for a line, probably I want it drawn no? :-)