Code shown in the "Default – MathJax 3" tab of the section on "Loading MathJax" at https://facelessuser.github.io/pymdown-extensions/extensions/arithmatex/#loading-mathjax

gistfile1.txt

window.MathJax = {
  options: {
    ignoreHtmlClass: 'tex2jax_ignore',
    processHtmlClass: 'tex2jax_process',
    renderActions: {
      find: [10, function (doc) {
        for (const node of document.querySelectorAll('script[type^="math/tex"]')) {
          const display = !!node.type.match(/; *mode=display/);
          const math = new doc.options.MathItem(node.textContent, doc.inputJax[0], display);
          const text = document.createTextNode('');
          const sibling = node.previousElementSibling;
          node.parentNode.replaceChild(text, node);
          math.start = {node: text, delim: '', n: 0};
          math.end = {node: text, delim: '', n: 0};
          doc.math.push(math);
          if (sibling && sibling.matches('.MathJax_Preview')) {
            sibling.parentNode.removeChild(sibling);
          }
        }
      }, '']
    }
  }
};