Why in the following code I see the whole page at once ? Thanks !

HTML:

<div></div>

CSS:

div {
    width: 100px;
    height: 300px;
    border: 1px solid black;
    text-align: center;
}

Javascript:

$(function() {
    for (var i=0; i<15; i++) {
        sleep(100);
        $("div").append("<span>--- " + i + " ---<br /></span>");
    }

    function sleep(milliseconds) {
        var start = new Date().getTime();
        for (var i = 0; i < 1e7; i++) {
            if ((new Date().getTime() - start) > milliseconds){
                break;
            }
        }
    }
});