The technique works for both fills and strokes, and is used by Vello for both, including for the text rendering. One of the nice things we found is that lowering Euler spirals to arcs works particularly well, and you need many fewer arc segments than line segments to accurately approximate a smooth curve. We haven't plumbed arcs all the way through the rendering pipeline yet, but it's planned; the math for rendering arcs is not that much more complicated than lines.