`{{< placeholder >}}` emits an image with no alt attribute

Comienza con src/resources/extensions/quarto/placeholder/placeholder.lua en las líneas 57–61, donde el shortcode llama a pandoc.Image, y revisa docs/authoring/placeholder.qmd para localizar los ejemplos afectados. Renderiza la reproducción QMD proporcionada a HTML y comprueba la regla de texto alternativo de imágenes con axe. Se considera terminado cuando los shortcodes placeholder sin contenido producen un atributo alt descriptivo basado en sus dimensiones y los ejemplos afectados de quarto.org ya no fallan la auditoría.

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

A bare {{< placeholder >}} in an HTML document produces an <img> element with no alt attribute. The output fails WCAG 2.0 A, SC 1.1.1 (Non-text Content). The axe rule image-alt reports it as a critical violation.

The attribute is absent, not empty. This difference is important. alt="" is valid HTML. It marks an image as decorative, and it passes image-alt. A missing attribute fails the rule.

The shortcode builds the image with pandoc.Image({}, result). The first argument holds the caption inlines, and it is always empty. No alt attribute reaches the output.

https://github.com/quarto-dev/quarto-cli/blob/abc6a78ed68f9e8bc9d54e27851093bd687a1cb7/src/resources/extensions/quarto/placeholder/placeholder.lua#L57-L61

An author can add alt text today with markdown image syntax and the fig-alt attribute:

![]({{< placeholder 400 200 >}}){fig-alt="Placeholder image"}

This affects quarto.org

An audit of quarto.org with axe found two of these images on docs/authoring/placeholder.qmd. Both are critical image-alt violations.

That page documents this shortcode, so the fig-alt workaround above is not available there. The workaround changes the examples, and the examples are what the page teaches.

Steps to reproduce

---
title: Placeholder
format: html
---

{{< placeholder 400 200 >}}

Render the document to HTML. Then read the alt attribute of the <img> element.

Actual behavior

The <img> element has no alt attribute:

<img src="data:image/png;base64,iVBORw0KGgoAAAANSU…" class="img-fluid">

Expected behavior

The shortcode writes a default alt string when the author gives none.

The shortcode already knows the width and the height. It also draws both values into the image as text:

local size_str = tostring(width) .. " x " .. tostring(height)

A default such as alt="Placeholder image, 400 by 200" therefore needs no change from the author. Every existing {{< placeholder >}} then passes the rule.

An empty alt="" also passes image-alt. A descriptive default is better here, for two reasons. First, the image contains text, and an empty alt states that the image conveys nothing. Second, the pages that use this shortcode use it to demonstrate a size or a layout. An empty alt maps the image to the presentation role, so assistive technology does not announce it. A screen reader user then loses the demonstration.

Your environment

• Positron 1.124.0
• macOS 26.5.2 (build 25F84)

Quarto check output

Quarto 1.10.18
[✓] Checking environment information...
      Quarto cache location: /Users/charlottewickham/Library/Caches/quarto
[✓] Checking versions of quarto binary dependencies...
      Pandoc version 3.10.0: OK
      Dart Sass version 1.101.0: OK
      Deno version 2.7.14: OK
      Typst version 0.15.1: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
      Version: 1.10.18
      Path: /Applications/quarto/bin
[✓] Checking tools....................OK
      TinyTeX: v2026.04
      Chrome Headless Shell: 150.0.7871.115
      VeraPDF: 1.28.2
[✓] Checking LaTeX....................OK
      Using: TinyTex
      Path: /Users/charlottewickham/Library/TinyTeX/bin/universal-darwin
      Version: 2026
[✓] Checking Chrome Headless....................OK
      Using: Chrome Headless Shell installed by Quarto
      Path: /Users/charlottewickham/Library/Application Support/quarto/chrome-headless-shell/chrome-headless-shell-mac-arm64/chrome-headless-shell
      Version: 150.0.7871.115
[✓] Checking basic markdown render....OK
[✓] Checking R installation...........OK
      Version: 4.6.0
      Path: /Library/Frameworks/R.framework/Versions/4.6/Resources
      LibPaths:
        - /Users/charlottewickham/Library/R/arm64/4.6/library
        - /Library/Frameworks/R.framework/Versions/4.6/Resources/library
      knitr: 1.51
      rmarkdown: 2.31
[✓] Checking Knitr engine render......OK
[✓] Checking Python 3 installation....OK
      Version: 3.12.2
      Path: /Users/charlottewickham/.pyenv/versions/3.12.2/bin/python3
      Jupyter: 5.9.1
      Kernels: python3
[✓] Checking Jupyter engine render....OK
[✓] Checking Julia installation...

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AI helped investigate this issue, grounded in a local clone of quarto-cli (see CONTRIBUTING.md, "Using AI tools to investigate").