<!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="font-size: 2.5rem">
    This text should all be autosized to 40px computed font-size; the span below shouldn't affect this.
    <span>
        Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.
    </span>
    Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.
</div>

</body>
</html>