var overlay = document.createElement('div');
var styles = {
    position: 'fixed',
    left: '0px',
    top: '0px',
    width: '100%',
    height: '100%',
    backgroundColor: 'rgba(255,255,255,0.75)',
    zIndex: 9999
};
for (var key in styles) {
    overlay.style[key] = styles[key];
}
document.body.appendChild(overlay);