If I have a map of three countries, and their borders form the outline of a sea, is their a way with SVG to give that bound area attributes and style?
Another example might be three triangles which form a third triangle in the center (like the triforce). If someone wanted to make that middle empty area glow (or whatever)...
With the triangles I'd say just draw the middle triangle 'again', or actually before your draw the 3 triangles. For the sea any simple shape fully covered by the 3 countries will do.
No, you would have to compute the bound shape and create that as a separate object.
However, let's say that you wanted to draw a map of the world. You could create a rectangle that was completely blue (for the water), and then draw all the countries on top of that. Then you wouldn't have to compute separate objects for all the various bodies of water.