Marker element percentages are missing reference values #64
Description
The current version of SVG2 document adds percentages as valid values for 'markerWidth', 'markerHeight', 'refX' and 'refY' in the <marker> element [1]. However, it doesn't say which value the percentages are relative to.
'refX' and 'refY' could be relative to the marker view box, because they are in the same coordinate space as marker content. I'm not sure which value 'markerWidth' and 'markerHeight' should refer to (stroke-width or path length?).
[1] https://svgwg.org/svg2-draft/painting.html#MarkerElement