$('[data-src]').wrap($('<div />').addClass('preload-highlighter'));
$('body').append(`
    <style>
        .preload-highlighter { position: relative; }
        .preload-highlighter:after {
            content: '';
            display: block;
            position: absolute;
            top:0;
            left: 0;
            right: 0;
            bottom: 0;
            background: rgba(255,0,0,.2);
            border: 1px solid #f00;
        }
    </style>
`);