CodeMirror
コピペでそのまま動きます
<link rel=stylesheet href="http://codemirror.net/lib/codemirror.css">
<script src="http://codemirror.net/lib/codemirror.js"></script>
<script src="http://codemirror.net/mode/xml/xml.js"></script>
<textarea id="txt">
<div>aa</div>
</textarea>
<script>
var txt = document.getElementById("txt");
var editor = CodeMirror.fromTextArea(txt, {
lineNumbers: true
,lineWrapping: true
});
editor.setSize("100%", "100%");
</script>