<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width">
<title>MathJax</title>
<script>
window.MathJax = {
tex: {
inlineMath: [['$', '$'], ['\\(', '\\)']],
},
loader: {load: ['ui/safe']},
};
</script>
<script id="MathJax-script" async src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-svg-full.js"></script>
</head>
<body>
$\longrightleftharpoons$
</body>
</html>
Please have a look at https://jsbin.com/gapezoravi/1/edit?html,js,console,output, where the upper arrow is smaller than the lower one. This is a strange side-effect of loading ui-safe.
This should solve mhchem/MathJax-mhchem#21.