Oh right, the image size the site uses is a bit weird so 100% would be 520x390px, I think the website might not save the images as png form at all. The browser renders the images based on the size of the browser window so the size varies between like 260x195(50%) and 840x630(~161.5%). 1280x960 would be ~246.2% and 1920x1440 ~369.2%. So yeah, I should probably make a proper export window that would ask for the resolution in pixels....
As for what would actually be in the final image, I think there are some very minor differences in the rendering systems of the site and on the client so the pixels near the edges of the lines might differ slightly between the two. Rendering at a higher resolution than what the site uses and scaling down with another program using e.g. bicubic scaling could give the closest result, although sometimes the renderer on the site causes moiré like patterns of its own so I don't think a perfect solution exists.
Replay preview is definitely a good idea, another window that would have a slider, play/pause and stop buttons, replay speed option and the canvas. then it would draw line segments and sleep for the rest of the frame before drawing the next batch. Should be possible. Would make animating things in dod replays easier.