XML-based languages for music have constraints not applicable to typical XML applications such as for standard text documents or data sets. Music contains numerous simultaneous events across several dimensions, including time. The Document Model for a piece of music would thus look very different from serialised text documents. Most existing XML-based music markup languages mark music typography, following the print traditions of music scores. A general music markup language should include much more than mere print. Some of the challenges designing an XML-based markup language for music are considered. An SVG-based Music Symbol Design Grid is proposed to meet the challenge of music typology. An XML-based Music Symbol Language is used to design symbols on this grid. Resulting symbols are positioned in 3D Music Space, which is introduced to address the challenge of topography.