<html xmlns="http://www.w3.org/1999/xhtml" xmlns:m="http://www.w3.org/1998/Math/MathML">
<head>
<style id="s">
[class="fx"] { position: fixed; }
</style>
</head>
<body onload="document.getElementById('s').textContent = '* { color: magenta; }'">
<m:mrow><m:mrow class="fx" /><span class="fx" /></m:mrow>
</body>
</html>