Default `documentclass` produces PDFs with no tagged headings

I have:

• searched the issue tracker for similar issues
• installed the latest version of Quarto CLI
• formatted my issue following the Bug Reports guide

Bug description

When a user sets pdf-standard: ua-2 on a format: pdf document,
Quarto correctly emits \DocumentMetadata{tagging=on, pdfstandard=ua-2, ...}
and veraPDF validates the result as PASSED against UA-2.

However, the PDF contains no tagged headings — \section and
\subsection collapse to plain <text> / <P> in the structure tree.

The default document classes scrartcl and scrbook are listed as incompatible on the LaTeX tagging project's
tagging status page.

Steps to reproduce

hello.qmd:

---
title: "Hello, Tagged World"
author: "Your Name"
lang: en-US
format:
  pdf:
    pdf-standard: ua-2
---

## Introduction

Body paragraph.

### A subsection

More body.

Render with quarto render hello.qmd, then look for "Introduction" in the structure tree
with the in-repo pdf-tag-tree.ts tool:

quarto run tools/pdf-tag-tree.ts hello.pdf "Introduction"

Actual behavior

With documentclass: scrartcl (Quarto's format: pdf default) —
veraPDF validates PASSED UA-2, but the structure tree has no headings:

Found "Introduction" on page 1

=== STRUCTURE TREE (Page 1) ===

<Root>
  <Document>
    <Part>
      <P> (MCIDs: p21R_mc0)
      <Table>
        <TR>
          <TD> (MCIDs: p21R_mc1)
    <Part>
      <P> (MCIDs: p21R_mc2)  # <-- found
    <Part>
      <P> (MCIDs: p21R_mc3)
    <Part>
      <P> (MCIDs: p21R_mc4)
    <Part>
      <P> (MCIDs: p21R_mc5)
    <Part>
      <L>
        <LI>
          <Lbl> (MCIDs: p21R_mc6)
          <LBody>
            <Part>
              <P> (MCIDs: p21R_mc7)
        ...

=== TEXT ITEMS CONTAINING "Introduction" ===

Text: "Introduction"
  MCID: p21R_mc2
  Tag path: Root > Document > Part > P

The heading "Introduction" is tagged as a plain <P> — no heading
tag, no <Sect> anywhere in the tree.

Expected behavior

## Introduction should tag as <H2> inside a <Sect> grouping
element; ### A subsection as <H3> inside a nested <Sect>.

E.g. with documentclass: article — same source, same veraPDF PASSED,
but real heading structure:

Found "Introduction" on page 1

=== STRUCTURE TREE (Page 1) ===

<Root>
  <Document>
    <Part>
      <P>
        <P> (MCIDs: p21R_mc0)
      <P> (MCIDs: p21R_mc1)
    <Sect>
      <H2> (MCIDs: p21R_mc2)  # <-- found
      <Part>
        <P> (MCIDs: p21R_mc3)
      <Sect>
        <H3> (MCIDs: p21R_mc4)
        <Part>
          <P> (MCIDs: p21R_mc5)
        <Part>
          <L>
            <LI>
              <Lbl> (MCIDs: p21R_mc6)
              <LBody>
                <Part>
                  <P> (MCIDs: p21R_mc7)
        ...

=== TEXT ITEMS CONTAINING "Introduction" ===

Text: "Introduction"
  MCID: p21R_mc2
  Tag path: Root > Document > Sect > H2

Your environment

No response

Quarto check output

$ quarto check
Quarto 1.9.36

[✓] Checking versions of quarto binary dependencies...
Pandoc version 3.8.3: OK
Dart Sass version 1.87.0: OK
Deno version 2.4.5: OK
Typst version 0.14.2: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking tools....................OK
TinyTeX: v2026.04
VeraPDF: 1.28.2
[✓] Checking LaTeX....................OK
Using: TinyTex
Path: /Users/.../TinyTeX/bin/universal-darwin
Version: 2026

Reproduce the issue with hello.qmd using the default scrartcl class, then compare the structure tree from tools/pdf-tag-tree.ts with the article-class result. Trace how the PDF output handles section headings and verify that the rendered document contains H2 and H3 tags nested in Sect elements while still passing veraPDF UA-2 validation.