Columns:

1. An external SVG references an external png image.
2. An external SVG references another external SVG which references an external png image.
3. An external SVG references another external SVG which references a data uri png image.
4. A data uri SVG references a data uri png image.
5. An external SVG references itself.
6. An external SVG references another external SVG which references the original SVG.