Bug in axe accessibility checks for revealjs formats

[paytonej]

Description

Quarto >1.8 has integrated support for axe-core accessibility checking html. The docs say that the axe YAML metadata configuration can be added to HTML formats (html, dashboard, and revealjs).

However, adding axe to the yaml for the revealjs format results in an error in compiling the sass themes. The error is:

ERROR: Theme file compilation failed:

Error: Undefined variable.
   ╷
67 │   border: 1px solid $body-color;

Line 67 defining the border is in quarto.js This suggests that theme file sass compilation is occurring after quarto.js inclusion, though it needs to happen before.

Accessibility enhancements should be high-priority due to new rules taking effect in April 2026 under Title II of the Americans with Disabilities Act and the requirements of the European Accessibility Act (EAA), which went into effect in June 2025. Many users are now liable for the documents they render meeting these requirements.

Related discussion items:

• Ensure Quarto Meets WCAG 2.1 AA Standards #12644
• Increase options/guidance for compliance with Web Content Accessibility Guidelines (WCAG) in revealjs HTML output #8621
• https://github.com/orgs/quarto-dev/discussions/11847

Minimal (not) working example. Change format to "html" to create something that renders appropriately.

---
title: "Axe Causes Sass Compilation Error"
date: today
format: 
  revealjs:
    axe: true
---

## Axe test.

This violates contrast rules: [insufficient contrast.]{style="color: #eee"}.

This test works with the format as `html` but not with the format as `revealjs`.

## For revealjs, axe causes theme file compilations to fail.

The resulting error is:

```{bash}
ERROR: Theme file compilation failed:

Error: Undefined variable.
   ╷
67 │   border: 1px solid $body-color;
```

Line 67 defining the border is in `quarto.js`

This suggests that theme file sass compilation is occurring after quarto.js inclusion, though it needs to happen before.

## The end

Bug found in Quarto 1.8.26 on 10 December 2025

Originally posted by @paytonej in #13780

[cderv]

Thanks for reporting this. I looked into it and found the root cause.

Root Cause

The axe accessibility feature adds a SASS bundle with a hardcoded dependency: "bootstrap":

quarto-cli/src/format/html/format-html-axe.ts

Lines 28 to 31 in b323477

	"sass-bundles": [
	{
	key: "axe",
	dependency: "bootstrap",

The SCSS rules use Bootstrap variables like $body-color and $link-color:

quarto-cli/src/format/html/format-html-axe.ts

Lines 37 to 58 in b323477

	rules: `
	body div.quarto-axe-report {
	position: fixed;
	bottom: 3rem;
	right: 3rem;
	padding: 1rem;
	border: 1px solid $body-color;
	}
	.quarto-axe-violation-help { padding-left: 0.5rem; }
	.quarto-axe-violation-selector { padding-left: 1rem; }
	.quarto-axe-violation-target {
	padding: 0.5rem;
	color: $link-color;
	text-decoration: underline;
	cursor: pointer;
	}
	.quarto-axe-hover-highlight {
	background-color: red;
	border: 1px solid $body-color;
	}`,

How SASS bundles work

Quarto groups SASS bundles by their dependency field and compiles each group separately. Bundles in the same group share SCSS variable context.

• HTML format has a "bootstrap" bundle that defines $body-color, $link-color, etc.
• Revealjs format has a "reveal-theme" bundle (not "bootstrap")

When axe is used with revealjs:

1. Axe bundle has dependency: "bootstrap"
2. There's no bootstrap bundle in revealjs - only reveal-theme
3. Axe compiles alone without any variable definitions
4. Compilation fails: Undefined variable $body-color

Why this could work

Revealjs defines the same variable names in its own SCSS:

quarto-cli/src/resources/formats/revealjs/quarto.scss

Lines 29 to 41 in b323477

	// main colors
	$body-bg: #fff !default;
	$body-color: #222 !default;
	$text-muted: lighten($body-color, 30%) !default;
	// grey colors (like in bootstrap)
	$gray-200: #e9ecef !default;
	$gray-100: #f8f9fa !default;
	$gray-900: #212529 !default;
	// link colors
	$primary: #2a76dd !default;
	$link-color: $primary !default;

So we could make the dependency format-aware using the existing isRevealjsOutput() function - use "reveal-theme" for revealjs, "bootstrap" for html/dashboard.

[cderv]

While implementing the fix, I found a deeper issue with how sass-bundles work for revealjs.

Making the dependency format-aware (dependency: "reveal-theme" for revealjs) doesn't fully solve it. The sass-bundles system in pandoc-html.ts groups bundles by dependency and compiles them together:

quarto-cli/src/command/render/pandoc-html.ts

Lines 62 to 77 in b323477

	// groups the bundles by dependency name
	const group = (
	bundles: SassBundleWithBrand[],
	groupedBundles: Record<string, SassBundleWithBrand[]>,
	) => {
	bundles.forEach((bundle) => {
	if (!groupedBundles[bundle.dependency]) {
	groupedBundles[bundle.dependency] = [];
	}
	groupedBundles[bundle.dependency].push(bundle);
	});
	};
	// group available sass bundles
	if (extras?.["html"]?.[kSassBundles]) {
	group(extras["html"][kSassBundles], mergedBundles);

For HTML+Bootstrap, the axe bundle compiles together with the bootstrap theme bundle, which provides the framework layer defining $body-color etc.

For revealjs, the theme compiles separately in format-reveal-theme.ts:

quarto-cli/src/format/reveal/format-reveal-theme.ts

Lines 186 to 198 in b323477

	// create sass bundle layers
	const bundleLayers: SassBundleLayers = {
	key: "reveal-theme",
	user: userLayers,
	quarto: mergeLayers(
	...quartoLayers,
	),
	framework: revealFrameworkLayer(revealSrcDir),
	loadPaths,
	};
	// compile sass
	const css = await compileSass([bundleLayers], project);

So an axe bundle with dependency: "reveal-theme" compiles standalone through pandoc-html.ts with no framework layer - even though revealjs defines these variables in its own compilation path.

The pragmatic fix is to add !default fallback values to the axe bundle. For HTML+Bootstrap, the framework layer overrides them. For revealjs, the fallbacks are used - meaning the axe report won't adapt to custom theme colors, but it will work.

A proper fix would require injecting axe layers into format-reveal-theme.ts compilation, or making revealjs consume sass-bundles like Bootstrap does. We could track that separately.