also consider the case of appendix titles in (div) wrappers

js/appendix.js

@@ -1,9 +1,23 @@
-// find <h[1-6] class="appendix"> and create a new <div> next to the parent
+// find <h[1-6] class="appendix"> and either add the class 'appendix' to
+// wrappers of appendix elements or create a new <div> next to the parent
 // element to hold all appendix elements
 (d => {
   const h = d.querySelector([1, 2, 3, 4, 5, 6].map(i => `h${i}.appendix`).join(','));
   if (!h) return;
   h.classList.remove('appendix');
+  // if h is in a wrapper whose next sibling has the same class, simply add class to wrappers
+  const p1 = h.parentNode; let p2 = p1, wrapper = false;
+  while (p1) {
+    p2 = p2.nextElementSibling;
+    if (!p2 || p1.tagName !== p2.tagName || p1.className !== p2.className) break;
+    p2.classList.add('appendix');
+    wrapper = true;
+  }
+  if (wrapper) {
+    p1.classList.add('appendix');
+    return;
+  }
+  // create a new div instead
   const a = d.createElement('div');
   a.className = 'appendix';
   a.append(h.cloneNode(true));