PDF validation should fail with no alt text

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

Bug description

Came across this example while invesitgating #14315.

The intermediate .tex has no alt attribute on the image:

\pandocbounded{\includegraphics[keepaspectratio]{alt-text_files/mediabag/quarto.png}}

There is a warning:

WARN: PDF accessibility: Missing alt text for image(s): alt-text_files/mediabag/quarto.png. Add alt text using fig-alt in YAML or {fig-alt="description"} on the image for PDF/UA compliance.

But, ultimately the validation passes:

[verapdf]: Validating alt-text.pdf against PDF/UA2...PASSED

I would expect this example to FAIL.

Is there some kind of fallback alt text being used? If so, I think we need to point it out more clearly in the warning, that in fact the passing validation should not be taken at face value.

Steps to reproduce

alt-text.qmd:

---
title: "Alt text"
format: 
  pdf:
    pdf-standard: ua-2
    keep-tex: true
---

![](https://quarto.org/quarto.png){fig-alt="Quarto logo"}

quarto render alt-text.qmd

Actual behavior

pandoc 
  to: latex
  output-file: alt-text.tex
  standalone: true
  variables:
    pdfstandard:
      standards:
        - ua-2
      tagging: true
    graphics: true
    tables: true
  pdf-engine: lualatex
  default-image-extension: pdf
  
metadata
  documentclass: scrartcl
  classoption:
    - DIV=11
    - numbers=noendperiod
  papersize: letter
  header-includes:
    - \KOMAoption{captions}{tableheading}
  pdf-standard-applied:
    - ua-2
  block-headings: true
  engines:
    - path: /Applications/quarto/share/extension-subtrees/julia-engine/_extensions/julia-engine/julia-engine.js
  title: Alt text
  

Rendering PDF
running lualatex - 1
  This is LuaHBTeX, Version 1.22.0 (TeX Live 2025) 
   restricted system commands enabled.
  
WARN: PDF accessibility: Missing alt text for image(s): alt-text_files/mediabag/quarto.png. Add alt text using fig-alt in YAML or {fig-alt="description"} on the image for PDF/UA compliance.

WARN: PDF accessibility: Document language not set. Add 'lang: en' (or appropriate language) to document metadata for PDF/UA compliance.

running lualatex - 2
  This is LuaHBTeX, Version 1.22.0 (TeX Live 2025) 
   restricted system commands enabled.
  

[verapdf]: Validating alt-text.pdf against PDF/UA2...PASSED

Output created: alt-text.pdf

Expected behavior

A FAIL for the validation.

Your environment

No response

Quarto check output

Quarto 1.9.36
[✓] Checking environment information...
      Quarto cache location: /Users/charlottewickham/Library/Caches/quarto
[✓] 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 Quarto installation......OK
      Version: 1.9.36
      Path: /Applications/quarto/bin

[✓] Checking tools....................OK
      TinyTeX: v2025.12
      VeraPDF: 1.28.2
      Chromium: (not installed)
      Chrome Headless Shell: (not installed)

[✓] Checking LaTeX....................OK
      Using: TinyTex
      Path: /Users/charlottewickham/Library/TinyTeX/bin/universal-darwin
      Version: 2025

[✓] Checking Chrome Headless....................OK
      Using: Chrome found on system
      Path: /Applications/Google Chrome.app/Contents/MacOS/Google Chrome
      Source: MacOS known location

[✓] Checking basic markdown render....OK

[✓] Checking R installation...........OK
      Version: 4.4.3
      Path: /Library/Frameworks/R.framework/Versions/4.4-arm64/Resources
      LibPaths:
        - /Users/charlottewickham/Library/R/arm64/4.4/library
        - /Library/Frameworks/R.framework/Versions/4.4-arm64/Resources/library
      knitr: 1.45
      rmarkdown: 2.26

[✓] Checking Knitr engine render......OK

[✓] Checking Python 3 installation....OK
      Version: 3.13.2
      Path: /Users/charlottewickham/.local/share/uv/python/cpython-3.13.2-macos-aarch64-none/bin/python
      Jupyter: (None)

      Jupyter is not available in this Python installation.
      Install with python3 -m pip install jupyter

[✓] Checking Julia installation...