<html>
<body>
<div id="box" style='border: 1px solid #000; height: 1000px;'>1000-pixel box</div>
<iframe id="iframe" src="data:text/html,<body><div style='border: 1px solid %23000; height: 5000px;'>5000-pixel box</div><button id='button'>Test Button</button></button></body>"></iframe>
</body>
</html>