<!DOCTYPE html>
<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=utf-8" />
<title>ГГ</title>
<style>
* {
margin:0;
padding: 0;
outline: none;
}
input[value = "+"] {
height: 20px;
width: 20px;
}
</style>
</head>
<body>
<div>
<input type="button" id="add" value="+" onclick="var parent = this.parentNode;var clone = this.cloneNode(true);parent.appendChild(clone);">
</div>
</body>
</html>