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quarto-dev
GitHub Repository: quarto-dev/quarto-cli
 Path: blob/main/llm-docs/axe-accessibility-architecture.md
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---
main_commit: ee0f68be1
analyzed_date: 2026-02-27
key_files:
  - src/format/html/format-html.ts
  - src/format/html/format-html-axe.ts
  - src/resources/formats/html/axe/axe-check.js
  - src/resources/formats/html/axe/axe-check.scss
  - src/resources/formats/revealjs/axe/axe-check.scss
  - src/resources/formats/dashboard/axe/axe-check.scss
---

Axe Accessibility Checking Architecture

Quarto's axe-core integration spans three layers: build-time TypeScript, compile-time SCSS, and runtime JavaScript. Each layer operates at a fundamentally different stage, which is why they detect formats differently.

Three-Layer Overview

Layer 1: Build-time TypeScript (format detection + dependency injection)

File: src/format/html/format-html.ts

TypeScript code detects the output format and conditionally injects axe-check.js + axe-check.css as a FormatDependency. The format detection uses the Quarto format system (isHtmlOutput, isRevealJsOutput, isDashboardOutput).

Key responsibilities:

  • Read axe option from document metadata

  • Inject axe files as FormatDependency (copies JS/CSS into output)

  • Encode options as base64 JSON into a <script> tag

The base64 encoding is a defensive pattern that prevents JSON containing </script> from breaking the HTML parser.

Layer 2: Compile-time SCSS (format-specific styling)

Files:

  • src/resources/formats/html/axe/axe-check.scss — HTML/dashboard styles

  • src/resources/formats/revealjs/axe/axe-check.scss — RevealJS-specific styles

SCSS files provide format-specific visual styling. RevealJS compiles its sass-bundles separately from the theme, so theme variables aren't in scope. The CSS custom property bridge (--quarto-axe-* variables set in HTML themes, consumed in RevealJS via var() with !default fallbacks) works around this architectural constraint.

Layer 3: Runtime JavaScript (scanning + reporting)

File: src/resources/formats/html/axe/axe-check.js

Single JS file handles all formats at runtime. Format detection uses DOM inspection:

  • typeof Reveal !== "undefined" → RevealJS

  • document.body.classList.contains("quarto-dashboard") → Dashboard

  • Otherwise → standard HTML

Key classes:

  • QuartoAxeChecker — Orchestrates scanning. Loads axe-core from CDN, runs scans, creates reporters.

  • QuartoAxeDocumentReporter — Format-specific DOM report (overlay, slide, or offcanvas)

  • QuartoAxeConsoleReporter — Logs violations to browser console

  • QuartoAxeJsonReporter — Dumps full axe result as JSON to console

Format-Specific Behavior

HTML (standard)

  • Report: overlay appended to <main> (or <body>)

  • Interaction: hover highlights violation elements

  • Rescan: none (static page)

RevealJS

  • Report: dedicated <section> slide appended to .reveal .slides

  • Interaction: click navigates to the slide containing the violation

  • Scan prep: temporarily removes hidden/aria-hidden from all slides so axe can inspect them

  • Rescan: none (slides are static)

Dashboard

  • Report: Bootstrap offcanvas sidebar with toggle button

  • Interaction: hover highlights violation elements

  • Rescan: triggered by shown.bs.tab (page/card tabs), popstate (back/forward), bslib.sidebar (sidebar toggle)

  • Generation counter prevents stale scan results from overwriting newer ones

Adding a New HTML Format

If a new HTML-based format is added that needs axe support:

  1. TypeScript (format-html.ts): Add format detection to the axe dependency injection logic

  2. SCSS: Create format-specific styles if the report placement differs. If the format uses Bootstrap themes, the CSS custom property bridge handles colors automatically.

  3. JavaScript (axe-check.js):

    • Add format detection in QuartoAxeDocumentReporter.report() — this determines which createReport*() method is called

    • Implement a createReport*() method for the format's DOM structure

    • If the format has dynamic content changes, add rescan triggers in setupDashboardRescan() (or create a format-specific equivalent)

  4. Tests: Add Playwright test fixtures (.qmd files in tests/docs/playwright/<format>/) and parameterized test cases in axe-accessibility.spec.ts

Key Design Decisions

CDN loading

axe-core (~600KB) is loaded from cdn.skypack.dev at runtime rather than bundled. This keeps the Quarto distribution small since axe is a dev-only feature. Tradeoff: requires internet, fails silently in offline/CSP environments.

Options encoding

Options are base64-encoded in a <script id="quarto-axe-checker-options"> tag. This prevents JSON containing </script> from breaking HTML parsing. The runtime decodes with atob().

Generation counter (dashboard rescan)

Rapid tab switches can queue multiple axe.run() calls. The scanGeneration counter ensures only the latest scan's results are displayed. Earlier scans complete but their results are discarded if a newer scan was started.

Known Limitations

  • SCSS compilation: RevealJS sass-bundles compile separately from themes. CSS custom properties bridge this gap, but direct SCSS variable sharing isn't possible without pipeline changes.

  • CDN dependency: No offline fallback. See quarto-cli-2u4f for documentation task.

  • Popstate delay: 50ms setTimeout for dashboard back/forward navigation. See quarto-cli-1fdf for improvement task.

  • Multi-main elements: createReportOverlay() uses document.querySelector('main') which returns the first match. Multiple <main> elements is invalid HTML, so this is acceptable.