function init() {
  var element = document.getElementById('body');
  var height = (navigator.userAgent.indexOf('MSIE') > -1 ? document.body.offsetHeight : self.innerHeight); 
 
  if (height > element.offsetHeight) {
    document.getElementById('mainContent').style.height = height - 273 + 'px';
  }
}

window.onload = init;
window.onresize = init;