<div id = 'dropZone'></div>

#dropZone
{
    background: gray;
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    z-index: 100;
    opacity: .8;
}

var dropZone = document.getElementById('dropZone');
dropZone.addEventListener('dragover', handleDragOver, false);
dropZone.addEventListener('drop', handleFileSelect, false);