MathJax v3 Configuration

In _includes/head_custom.html add, for example:

{% case page.math %}
     
  {% when "mathjax3" %}
    
    <script>
      MathJax = { 
        tex: { 
          tags: 'ams',
          packages: {'[+]': ['textmacros']},
        },
        loader: {
          load: ['[tex]/textmacros']
        }
      };
    </script>
    <script src="https://polyfill.io/v3/polyfill.min.js?features=es6"></script>
    <script type="text/javascript" id="MathJax-script" async
      src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-chtml.js">
    </script>
   
{% endcase %}

In the front matter of pages using MathJax v3 (or as a global front-matter default) add:

math: mathjax3

(The suggested field name math and the key mathjax3 can be replaced.)