How can I customise the browser's output for print/print preview?

<html>
<head>
<style id="styles" type="text/css">
@media print { .noprint { display:none; } }
</style>
<script type="text/javascript">
var x = Math.random();

if (x > .5) {
    var style = document.createElement('style');
    style.type = 'text/css';
    style.innerHTML = '@media print { .maybe_noprint { display:none; } }';
    document.getElementsByTagName('head')[0].appendChild(style);
}
</script>
</head>
<body>
<div class="noprint">This will never print.</div>
<span class="maybe_noprint">This may print depending on the value of x.</span>
</body>
</html>

<div id='div19' class='noprint'>
  ...
</div>

.noprint {
  display: none;
}

document.getElementById('div19').className='noprint';