Text-based formats like XML and JSON are good if most of your data is strings, but storing numeric data in decimal strings is both inefficient and inaccurate. That's why I think that HTML is an appropriate use of XML-like markup but perhaps vector images would need a true binary version.

However I agree with saagarjha that it is very easy to write code to emit SVG, which is nice.

And their text format is exceedingly useful if you're writing code to generate them.

One could conceivably make a binary format based on Messagepack or Flatbuffers for vector data, which would be much more efficient and eliminate rounding problems of converting between floats and decimal numbers in text representations.