Always redraw Mandelbrot set afresh on first load (slower, but reflects algorithm changes).
Draw ongoing updates to the Mandelbrot set, rather than pre-selecting a maximum number of steps.
Fix memory leak, at the expense of making the zoom code awful
Make much more impressive images, with dithering and a colour scheme that does not change with time, repeats or zoom
Make zoom boxing enclose user's rectangle, keeping the center the same (previous algorithm cut off part of the user's rectangle, and kept the top-left corner the same)
Output number of steps processed after each iteration.