<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html lang="en">
<head>
<title>title</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<script type="text/javascript">
window.onload = function() {
var input = document.createElement('input');
document.body.appendChild( input );
input.focus();
}
</script>
</head>
<body>
<input type="text" value="12">
</body>
</html>
работает?