jQuery
<!DOCTYPE html>
<html lang="ja-JP">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width,user-scalable=yes,initial-scale=1.0,minimum-scale=1.0,maximum-scale=1.5">
<title></title>
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script>
<script>
$(function(){
$("input").click(function(){
$("p#first").after("<h1>後ろに挿入</h1>");
});
});
</script>
</head>
<body>
<input type="button" value="click">
<p id="first">テキストテキスト</p>
<h1>後ろに挿入</h1>
</body>
</html>