Canvas fill browser window by setting viewport scale.

gistfile1.js

var screenScale = 1;
window.devicePixelRatio = window.devicePixelRatio || Math.round(screen.deviceXDPI*10 / screen.logicalXDPI)/10;

// Calc safe zone for pixels.
if( window.devicePixelRatio > 1 ) {

	var lScreenWidth = Math.max( screen.width, screen.height ) * window.devicePixelRatio;
	var lScreenHeight = Math.min( screen.width, screen.height ) * window.devicePixelRatio;
	
	var SAFE_ZONE_WIDTH = 2048;
	var SAFE_ZONE_HEIGHT = 1365;
	
	if (!isWebGLSupported()) {
		SAFE_ZONE_WIDTH/=2;
		SAFE_ZONE_HEIGHT/=2;
	}
	
	var lRatio = Math.max(1,Math.ceil(Math.min(lScreenWidth / SAFE_ZONE_WIDTH, lScreenHeight / SAFE_ZONE_HEIGHT)));
	
	screenScale = Math.round(100*lRatio / window.devicePixelRatio) / 100;
}

var head = document.getElementsByTagName('head')[0];
var meta = document.createElement('meta');
meta.setAttribute('name', 'viewport');
meta.setAttribute('content', 'initial-scale=' + scale + ', user-scalable=no, minimal-ui');
head.appendChild( meta );