fix(math): Requires page.math for Mathjax. Add integrity checks

Author: pham

_includes/foot/mathjax.html

@@ -1,2 +1,13 @@
-<script defer src="https://polyfill.io/v3/polyfill.min.js?features=es6"></script>
-<script async id="MathJax-script" src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js"></script>
+<script defer
+  src="https://cdnjs.cloudflare.com/polyfill/v3/polyfill.min.js?version=4.8.0"
+  integrity="sha384-3d4jRKquKl90C9aFG+eH4lPJmtbPHgACWHrp+VomFOxF8lzx2jxqeYkhpRg18UWC"
+  crossorigin="anonymous">
+</script>
+{%- if page.math -%}
+<script async
+  id="MathJax-script"
+  src="https://cdn.jsdelivr.net/npm/mathjax@4.1.0/tex-mml-chtml.js"
+  integrity="sha384-pTCnJ6SV7q3Uxsux1a1dYA7psHUmFQB178t6FtIE16r5QY5EibXwxLy9KRzBqM1w"
+  crossorigin="anonymous">
+</script>
+{%- endif %}

content/_session/parts/formatting.md

@@ -3,6 +3,17 @@
 
 [GitHub Markdown] syntax are supported including extras such as [Markdown Math].
 
+$$ x = {-b \pm \sqrt{b^2-4ac} \over 2a} $$
+
+Requires:
+
+```
+  ---
+  math: true
+  ---
+```
+
+
 ### Supported Header Tags
 Header tags from `h1` to `h6` are supported.
 Though `<h1>` is the largest, it is recommended that you should start with `<h2>` for main general headings.

content/_session/session-03.md

@@ -1,4 +1,5 @@
 ---
+math: true
 ---
 
 {% include_relative parts/formatting.md %}