1) it does count edge pixels - recognition is based on the shape of what you draw, so for the algorithm, a black picture is like a white one (but I admire your endurance to paint it all black)
2) server is under heavy load atm, it might drop some requests - just retry
There is a prior probability in the results - but it's hard to balance it so that latin characters don't dominate the results if you search something else. That is why I opted for the way it is now - this is a tool to find unicode characters, you have latin characters on your keyboard. It's not really meant for OCR
I would like to compile it on my own with emscripten. Did you use emconfigure or did you write you own compilation script? Couldn't find any pointers to that in the Git, but maybe I didn't look around long enough to find it. How did you deal with kpathsea?
There is a whole chapter on shape contexts in it, which I use with shapecatcher, too.