globalAlpha:

your browser does not support the canvas tag

var c = document.getElementById('myCanvas');
var ctx = c.getContext('2d');
ctx.fillStyle = 'red';
ctx.fillRect(20, 20, 75, 50);
ctx.globalAlpha = 0.5;
ctx.fillStyle = 'blue';
ctx.fillRect(50, 50, 75, 50);