<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<style>
div.flexbox {
display: flex;
flex-direction: column;
}
</style>
</head>
<body>
<div class="flexbox"><mo xmlns="http://www.w3.org/1998/Math/MathML"><mrow/></mo></div>
</body>
</html>