<!DOCTYPE html>
<html>
<head>

<meta name="viewport" content="width=800">
<style>
html { font-size: 16px; }
body { width: 800px; margin: 0; overflow-y: hidden; }
</style>

</head>
<body>

<div style="width: 320px; font-size: 1rem">
    This text should not be autosized, as this div is the lowest common ancestor of the root cluster, and this div is narrow.<br>
    Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.
    <div style="width: 800px; font-size: 2.5rem">
        This text should be autosized to 40px computed font size (16 * 800/320) since it's wider than the lowest common ancestor of its enclosing cluster so it becomes a cluster itself.<br>
        Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.
    </div>
</div>

</body>
</html>