Caption should not be used as fallback alt text for images in PDF/Typst

[gordonwoodhull]

Summary

When an image has a caption but no explicit fig-alt, Quarto silently copies the caption into the image's alt text attribute for LaTeX/PDF and Typst output. This was introduced as part of PDF/UA compliance work but is an accessibility anti-pattern.

The problem

Captions and alt text serve fundamentally different purposes:

• Caption: Visible text describing the figure's significance in context (e.g., "Figure 3: Comparison of methods A and B showing the 2x improvement")
• Alt text: Non-visible text describing what the image looks like for screen readers (e.g., "A bar chart with two bars labeled Method A and Method B, where B is approximately twice the height of A")

Using the caption as alt text violates WCAG guidance, which explicitly notes that when a caption is already present, the alt text should describe the image content rather than duplicate the caption. The current behavior silences PDF/UA validators without actually improving accessibility.

Affected code

The fallback logic exists in three places:

1. src/resources/filters/layout/latex.lua:335-337 (and :387-389, :672-674) — three separate code paths that copy the stringified caption into image.attributes["alt"] before clearing the caption for LaTeX rendering
2. src/resources/filters/quarto-post/typst.lua:228-232 — same pattern for Typst output

All introduced in commits a867c3c24 (Jan 9, 2026, "Add pdf-standard option with verapdf validation") and ba75b374f (Jan 10, 2026, "Add Typst image alt text for PDF/UA accessibility").

Note: explicit fig-alt works correctly

The explicit fig-alt propagation in src/resources/filters/quarto-pre/figures.lua:31-53 is fine — it correctly propagates user-specified alt text. The problem is only in the fallback when no fig-alt is provided.

Suggested fix

Remove the caption-as-alt fallback and instead either:

• Emit no alt text, letting PDF/UA validators flag it so authors add proper fig-alt
• Emit a build warning when an image has a caption but no fig-alt, prompting the author to add one