Placing some offset directional arrows along a path, take 4.
This method is definitely overkill. It calculates one offset point per three pixels along the original path, and then walks that list of points one at a time to ensure that the resulting arrows are roughly evenly spaced and roughly the same length, regardless of the angles of their curves. You could make this slower + more precise, or faster + less precise, by changing the number of pixels per step.
marker-segment and marker-pattern should make this easier eventually.