Your mathjax script tag isn’t closed properly:
<script type="text/javascript" id="MathJax-script" async
src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-chtml.js">
</head>
<body ... >
This means that the </head>, and the <body> and all of your page content is treated as text inside the <script> tag until you hit the </script> from your document.body.classList.remove("no-js");, and lost. (If you view page source in your browser you should be able to see this: the tags after your MathJax-script tag have no syntax highlighting.)
I don’t know where that <script> tag came from but you should generate add a </script> close tag after it to close it properly.
<script type="text/javascript" id="MathJax-script" async
src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-chtml.js"></script>