scaleToFit

myElement.style.position = "absolute";
myElement.style.top = "50%";
myElement.style.left = "50%";
scaleToFit();
    
function scaleToFit() {
    var availableHeight = window.innerHeight,
        availableWidth = window.innerWidth,
        scale = Math.min(availableWidth / myElement.width, availableHeight / myElement.height);

    myElement.style.webkitTransform = myElement.style.transform = "translate(-50%, -50%) scale("+scale+")";
}