Bug

dfn-index.js crashes with a TypeError when an exported <dfn> element appears in the <section id="abstract"> or any other unnumbered section (introductory sections, SotD, etc.).

Error

Error in handler for topic "toc": Cannot read properties of null (reading 'textContent')
  Plugin: sub:toc

In Safari/WebKit the full error is:

null is not an object (evaluating 'document.getElementById(e).closest("section:not(.notoc)").querySelector(".secno").textContent')

Root cause

dfn-index.js:appendSectionNumbers() iterates every term in the local dfn index and tries to read the section number of the section containing each dfn:

const sectionNumberEl = dfn
  .closest("section:not(.notoc)")
  .querySelector(".secno");
const secNum = `§${sectionNumberEl.textContent.trim()}`; // crashes if .secno is null

The <section id="abstract"> (and other special/introductory sections) never receive a .secno element because structure.js skips numbering for intro sections. So when a <dfn> in the abstract ends up in the index, .querySelector('.secno') returns null and the next line crashes.

Reproduction

<\!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8" />
  <title>Test</title>
  <script src="https://www.w3.org/Tools/respec/respec-w3c" class="remove" async></script>
  <script class="remove">
    var respecConfig = { specStatus: "ED", editors: [{ name: "Test" }] };
  </script>
</head>
<body>
  <section id="abstract">
    <p>This spec defines the <dfn>my-concept</dfn>.</p>
  </section>
  <section id="sotd"></section>
  <section id="index"></section>
</body>
</html>

Fix

Add a null guard in appendSectionNumbers():

const sectionNumberEl = dfn
  .closest("section:not(.notoc)")
  ?.querySelector(".secno");
if (\!sectionNumberEl) return null; // dfn in unnumbered section — skip

A PR with the fix and regression test is on the feat/validate-header-links branch.