You'll have to do it with JavaScript in most browsers, I think. And it won't work on iOS, which only allows programmatic use of focus() in an event handler of a genuine user interaction (such as a click or touch-related event).
Also, be aware that if there other inputs on the page, it's possible the user may focus on one of those before the document has completely loaded, in which case it is then very annoying for the user to find that the focus has been moved from under them when the load event does fire, so you should use the inputs' focus events to track this.
<div contenteditable="true" id="editable"></div>
<script type="text/javascript">
window.onload = function() {
document.getElementById("editable").focus();
};
</script>