[danik.js]

<p><button>Кнопка</button></p>
<p><button>Кнопка</button></p>
<p><button>Кнопка</button></p>
<script src="http://code.jquery.com/jquery-latest.min.js"></script>
<script>
    $('<div/>').css({
        position: 'fixed',
        top: 0,
        left: 0,
        right: 0,
        bottom: 0,
        background: 'black',
        opacity: 0.5
    }).appendTo(document.body);
</script>