addLoadEvent()

html

<div id="contentload">
  <p>Here goes...</p>
</div>

JavaScript

function addLoadEvent(func) {
    var oldonload = window.onload;
    if (typeof window.onload != 'function') {
        window.onload = func;
    } else {
        window.onload = function() {
            if (oldonload) {
                oldonload();
            }
            func();
        }
    }
}

function addContentOnLoad() {
  var contentElement = document.getElementById("contentload");
  contentElement.style.backgroundColor = 'yellow';
  contentElement.appendChild(document.createTextNode('Hungry'));
  contentElement.appendChild(document.createTextNode(' monkeys!'));
}
addLoadEvent(addContentOnLoad);