6.3. Simplify SVG
Mapnik only supports a subset of SVG when it comes to marker images so far.
When creating images with InkScape, make sure to save your images as "Plain SVG", not "InkScape SVG". Even with that some unsupported constructs may end upSVG files though.
The same is also true for SVG images from other sources.
When it comes to simple unknown attributes Mapnik will throw a warning, but the result will usually still come out OK. In other cases you may end up with weird looking results though. And even when everything looks fine in the end, the emitted warnings can still be annoying, and may hide other, more importantn warnings.
TODO: simple script to resolve the most common cases.