i.e. CSS expression

<html>
<style>
p {
border:1px solid red;
max-width:30em;
width:expression( 
    document.body.clientWidth > (500/12) * 
    parseInt(document.body.currentStyle.fontSize)?
        "30em":
        "auto" );
}
</style>
<body>
<p>
[alot of text]
</p>
</body>
</html>

reference : http://www.svendtofte.com/code/max_width_in_ie/