appendChild et insertBefore
by toubia95
HTML
<div id="editor" >
<pre contentEditable="true">1</pre>
<pre contentEditable="true">2</pre>
<pre contentEditable="true">3</pre>
</div>
JavaScript
//http://stackoverflow.com/questions/8771921/insert-sibling-node-in-js
var container = document.getElementById('editor'),
firstChild = container.childNodes[1];
if (container && firstChild) {
var newPre = document.createElement('pre');
newPre.innerHTML = "boom";
firstChild.parentNode.insertBefore(newPre, firstChild.nextSibling);
}