diff --git a/build-slides.sh b/build-slides.sh
index 3bd3b60..47b4a4a 100755
--- a/build-slides.sh
+++ b/build-slides.sh
@@ -52,12 +52,17 @@ document.querySelectorAll('code.language-mermaid').forEach(el => {
   div.textContent = el.textContent;
   el.closest('pre').replaceWith(div);
 });
-mermaid.initialize({ startOnLoad: false, theme: 'dark' });
-await mermaid.run();
+mermaid.initialize({ startOnLoad: true, theme: 'dark' });
 </script>"""
 
-content = open(path).read()
-open(path, 'w').write(content.replace('</body>', snippet + '\n</body>', 1))
+with open(path, encoding='utf-8') as f:
+    content = f.read()
+new_content = content.replace('</body>', snippet + '\n</body>', 1)
+if new_content == content:
+    print(f"Warning: </body> not found in {path}", file=sys.stderr)
+    sys.exit(1)
+with open(path, 'w', encoding='utf-8') as f:
+    f.write(new_content)
 PYEOF
 }