From b099499881af2eba0a6af31ada2bf90a5409dc12 Mon Sep 17 00:00:00 2001
From: Marcos Caceres <caceres_m@apple.com>
Date: Sat, 9 May 2026 14:23:15 +1000
Subject: [PATCH] fix(core/title): detect id="title" collision and use fallback

When another element already uses id="title", the spec's h1 now gets
id="document-title" instead of silently creating a duplicate ID. An
error is shown identifying the conflicting element.

Closes #4712
---
 src/core/title.js | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/src/core/title.js b/src/core/title.js
index 912df79c35..9d2c2b7b4e 100644
--- a/src/core/title.js
+++ b/src/core/title.js
@@ -51,7 +51,18 @@ export function run(conf) {
   }
 
   // Decorate the spec title
-  if (!h1Elem.id) h1Elem.id = "title";
+  if (!h1Elem.id) {
+    const existing = document.getElementById("title");
+    if (existing && existing !== h1Elem) {
+      const msg =
+        'Another element already uses `id="title"`. ' +
+        "The spec's `<h1>` will use a different id to avoid a collision.";
+      showError(msg, name, { elements: [existing] });
+      h1Elem.id = "document-title";
+    } else {
+      h1Elem.id = "title";
+    }
+  }
   h1Elem.classList.add("title");
 
   setDocumentTitle(conf, h1Elem);