Unless I misunderstand the question, I'd say Scalable Vector Graphics. A pretty decent format that's very badly supported in out-of-the-box browsers even though

Unless I misunderstand the question, I'd say Scalable Vector Graphics. A pretty decent format that's very badly supported in out-of-the-box browsers even though it's been in existance for several years. Consistent SVG support would allow a lot of server work to be shifted to the client.