Mainly because you forgot to allow for circles, bitmapped objects, arbitrary
polygons, external spatial indexers, etc. The server can be very flexible
but the client cannot, any finite client side scheme you implement will
not be sufficient. HTML+ defines polygon support with <FIGA> but it isn't
yet implemented anywhere.
There isn't any reason you can't do your own relative path munging in the
server, it has all the information it needs.
--sanders