var d = document;
var orgHeight = "";

function divHeight() {
var dheader = d.getElementById('header').clientHeight;
var dmainnav = d.getElementById('mainnav').clientHeight;
var dfooter = d.getElementById('footer').clientHeight;
	
if (orgHeight=="") orgHeight = d.getElementById('content').clientHeight;
	var myHeight = d.documentElement.clientHeight-dheader-dmainnav-dfooter-21;		//Abstand zw. mainnav und content
	(myHeight>=orgHeight)? d.getElementById('content').style.height = myHeight+'px': d.getElementById('content').style.height = orgHeight+'px';
}

divHeight();