I'm still improving Inkscape's marker support, and have discovered another deficiency in the spec:
11.6.2 "the marker's positive x-axis should point toward the angle bisector for the angle at the given vertex,"
Consider the attached SVG. The black line represents a path Its end point is at the bottom left of the drawing. Confusingly, all of the markers in Inkscape are designed to point from the end of the line towards its start, a convention I've followed here. The red lines represent the inner angle bisectors. The blue lines represent markers with positive x-axis pointing towards the angle bisector.
You will notice that the two arrows point in opposite directions. This is clearly undesirable. The correct action would be to use the inner angle bisector for the leftmost marker, and the outer for the rightmost marker.
A rule which accomplishes this is: If the clockwise angle from the incoming vector to the outgoing vector is greater than 180%, use the outer angle bisector, else use the inner angle bisector. Incoming vectors are those which are closer to the start of the path.
This is the rule I intend to implement in Inkscape.
