This repository contains a small polyfill for the MathML mfenced element. In order to use it, just load the mfenced.js script:
<html>
<head>
...
<script src="mfenced.js"></script>
...
</head>
...
</html>
At page load, all the mfenced elements will be converted into their equivalent expanded form. A function window.expandMathMLFencedElements is also defined and can be used to execute this conversion again later.