Revert

_includes/footer.html

@@ -17,5 +17,7 @@
 </div>
 
 
-<script id="MathJax-script" async src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js"></script>
+<script type="text/javascript"
+    src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.1/MathJax.js?config=TeX-AMS-MML_HTMLorMML">
+    </script>
 {% endif %}