@sbs: No need to implement png encoder from scratch. Just draw QR code in canvas and add simple option to export it. 
For example you can draw a canvas and change it to image really easy in JavaScript:
var canvas = document.querySelector("canvas#selector.here");
var data = canvas.toDataURL("image/png");
document.write('<img src="' + data + '"/>');
It’s much more simpler and allows to simple export in much more formats without any library for this!
