Save and Restore Global Alpha
Save and Restore does affect global alpha for HTML Canvas.
HTML
<canvas id='canvas' width='400' height='600'></canvas>JavaScript
var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
ctx.fillRect(0, 0, 600, 200);
ctx.save();
ctx.globalAlpha = 0.4;
ctx.fillRect(0, 200, 600, 200);
ctx.restore();
ctx.fillRect(0, 400, 600, 200);