diff --git a/_includes/custom-head.html b/_includes/custom-head.html
new file mode 100644
index 0000000..cb20434
--- /dev/null
+++ b/_includes/custom-head.html
@@ -0,0 +1,6 @@
+{% comment %}
+
+Any uncommented code in this file will be included just before the end of
+the document head.
+
+{% endcomment %}
diff --git a/_layouts/default.html b/_layouts/default.html
index dc94f63..0d0d4c1 100644
--- a/_layouts/default.html
+++ b/_layouts/default.html
@@ -10,6 +10,7 @@
       <link rel="icon" href="{{ site.favicon | relative_url }}">
       {%- endif %}
     <link rel="stylesheet" href="{{ "/assets/css/style.css?v=" | append: site.github.build_revision | relative_url }}">
+    {% include custom-head.html %}
   </head>
   <body>
     <div class="container-lg px-3 my-5 markdown-body">
diff --git a/_layouts/spec.html b/_layouts/spec.html
index de79b69..d79df42 100644
--- a/_layouts/spec.html
+++ b/_layouts/spec.html
@@ -167,6 +167,7 @@
         </style>
         <script src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-chtml.js" id="MathJax-script" defer></script>
         {%- endif %}
+        {% include custom-head.html %}
     </head>
     <body>
         <div id="primer-spec-top"></div>
