<canvas id="myCanvas" width="300" height="150" style="border:1px solid #d3d3d3;">
Your browser does not support the HTML5 canvas tag.
var c=document.getElementById("myCanvas");
var ctx=c.getContext("2d");
ctx.fillRect(20,20,75,50);
ctx.globalCompositeOperation="source-over";
ctx.fillRect(50,50,75,50);
ctx.fillRect(150,20,75,50);
ctx.globalCompositeOperation="destination-over";
ctx.fillRect(180,50,75,50);