I dunno, I always see transparency as the optimal background choice over a solid color, as it can be easily changed by applying a color background to the HTML page itself; conceivability is in the viewer's hands. Technically, the very background image could be set to the background as well with this method.
What I'm really after in my maps, though, is that it has only the things you need to know; where the starting point is, where the enemies are, where the water level is.. things like that which assist you to the optimal route or whatever. A background only pleases one's eyes and might make the viewer oversee crucial, small elements that could blend in.