Use MathJax with kramdown, without protecting in div's (as advertised!)

I'm trying to set up jekyll to display math properly.

The problem is nothing renders if I do not nest equations inside div tags to protect them from the markdown parser :(

---
layout: default
math:   true
---

# Using Mathjax

along markdown parsers is a real pain...

$$ a + b = c $$

doesn't show, though the following does:

<div> 
$$ x + y + c $$
</div>

The layout tests includes the following mathjax script through an include {% if page.math %}:

<script async src="https://cdn.jsdelivr.net/npm/mathjax@2/MathJax.js?config=TeX-AMS-MML_CHTML"></script>

I'm confused because:

jekyll pretends to use kramdown by default
kramdown
pretends to support mathjax by default with the above syntax.

There is already a lot on this topic, however much is outdated! Any help for a smooth and simple configuration would be greatly appreciated.

Solution

The problem is that kramdown's output is not compatible with mathjax v3 anymore.

You can check that kramdown does its job at nesting $$ a + b = c$$ inside the recommended script tags:

<script type="tex/math"> a = b + c </script>

However mathjax does not use these script tags anymore, as mentioned here (last bullet).

So having kramdown work with mathjax either requires to:

include a reliable source to mathjax 2: my attempts failed
include this complicated mathjax 3 configuration object before the mathjax script, passing it a function to look for those tags

MathJax = {
  options: {
    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('');
          node.parentNode.replaceChild(text, node);
          math.start = {node: text, delim: '', n: 0};
          math.end = {node: text, delim: '', n: 0};
          doc.math.push(math);
        }
      }, '']
    }
  }
};

Can't figure why this backward combatability instead of a simple option switch though...