That proved a little more convoluted than expected, but now works well.

resources/templates/edit-css.html

@@ -0,0 +1,22 @@
+{% extends "templates/base.html" %}
+
+{% block content %}
+        <div id="content" class="edit">
+            <form action="{{servlet-context}}/edit-css" method="POST">
+                <input type="hidden" name="page" value="{{page}}"/>
+                <textarea name="src" id="src" rows="25" cols="80">{{content}}</textarea>
+                <p class="widget">
+                    <label for="summary">What have you changed?</label>
+                    <input name="summary" id="summary" type="text"
+                        value="{%if exists%}{%else%}New file {{title}}{%endif%}" required/>
+                </p>
+                <p class="widget">
+                    <label for="submit">When you have finished editing</label>
+                    <input name="submit" id="submit" type="submit" class="action" value="Save!"/>
+                </p>
+            </form>
+        </div>
+        <script>
+          var simplemde = new SimpleMDE();
+        </script>
+{% endblock %}