From a19366a51233a43614b5bb7bc9edc62ef813c7f6 Mon Sep 17 00:00:00 2001
From: Anne van Kesteren <annevk@annevk.nl>
Date: Wed, 10 Jun 2026 04:59:00 -0700
Subject: [PATCH] headingoffset computation should not use the flat tree

Like the lang attribute it should use the composed tree. Or in other words, semantics should not be impacted by slotting.
---
 source | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/source b/source
index 640465da59f..6dc9d19de2d 100644
--- a/source
+++ b/source
@@ -20833,8 +20833,12 @@ interface <dfn interface>HTMLHeadingElement</dfn> : <span>HTMLElement</span> {
      and has a <code data-x="attr-headingreset">headingreset</code> attribute, then return
      <var>offset</var>.</p></li>
 
-     <li><p>Set <var>inclusiveAncestor</var> to the parent node of <var>inclusiveAncestor</var>
-     within the <span>flat tree</span>.</p></li>
+     <li><p>If <var>inclusiveAncestor</var>'s parent is a <span>shadow root</span>, then set
+     <var>inclusiveAncestor</var> to that <span>shadow root</span>'s <span
+     data-x="concept-DocumentFragment-host">host</span> and <span>continue</span>.</p></li>
+
+     <li><p>Set <var>inclusiveAncestor</var> to <var>inclusiveAncestor</var>'s <span>parent
+     element</span>.</p></li>
     </ol>
    </li>