function setContentMiddle() {
    var t = document.getElementById('content');
    var b = t.offsetHeight;
    var w = getClientHeight();
    var m = ((w - b) / 2 > 0) ? ((w - b) / 2) : 0;
    
    t.style.marginTop = m + 'px';
    }

function getClientHeight() {
    if(document.documentElement.clientHeight) {
        return document.documentElement.clientHeight;
    }
    else if(document.body.clientHeight) {
        return document.body.clientHeight;
    }
    else if(document.innerHeight) {
        return document.innerHeight;
    }
}

if(window.addEventListener) {
    window.addEventListener('DOMContentLoaded', setContentMiddle, false);
    window.addEventListener('resize', setContentMiddle, false);
}
else if(window.attachEvent) {
    window.attachEvent('onload', setContentMiddle);
    window.attachEvent('onresize', setContentMiddle);
}