js canvas alpha

var c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");
ctx.fillStyle = "red";
ctx.fillRect(20, 20, 75, 50);

// Turn transparency on
ctx.globalAlpha = 0.2;
ctx.fillStyle = "blue"; 
ctx.fillRect(50, 50, 75, 50); 
ctx.fillStyle = "green"; 
ctx.fillRect(80, 80, 75, 50);