Mermaid Diagrams Accessibility Skill

Canonical source: examples/MERMAIDACCESSIBILITYBEST_PRACTICES.md in mgifford/ACCESSIBILITY.md
This skill is derived from that file. When in doubt, the example is authoritative.

Apply these rules when authoring, generating, or reviewing Mermaid diagrams. Only load this skill if the project uses Mermaid diagrams.

Core Mandate

Accessible Mermaid content depends on four layers: (1) the diagram source and its accessibility metadata, (2) the exact Mermaid renderer and configuration, (3) the generated SVG/raster output, and (4) how the publishing platform embeds and exposes that output. A generated Mermaid diagram is non-text content unless the rendered nodes/relationships are proven to have useful semantics — test the final published output, not just the source or an editor preview.

Before authoring, decide whether a diagram is even needed — a heading, short list, table, or few sentences may communicate the information more clearly. Mermaid is useful when relationships, sequence, branching, hierarchy, timing, or spatial grouping materially improve understanding.

Severity Scale (this skill)

Critical: Use the Correct accTitle/accDescr Syntax — No %% Prefix

%% is the Mermaid comment prefix. Lines written as %%accTitle … or %%accDescr … are comments and are silently ignored — this produces no accessibility metadata at all while looking like it works.

# WRONG — these are Mermaid comments and do nothing
%%accTitle This is ignored as a comment
%%accDescr This is also ignored as a comment

Correct syntax uses accTitle: and accDescr: directly, with no comment prefix:

flowchart TD
    accTitle: Account recovery decision flow
    A[Submit email address] --> B{Account found?}
    B -->|Yes| C[Send recovery link]
    B -->|No| D[Show recovery help]

Single-line description uses a colon:

flowchart LR
    accTitle: Publishing workflow
    accDescr: Content moves from drafting through accessibility review to publication.
    A[Draft] --> B[Accessibility review] --> C[Publish]

For a multi-line description, omit the colon after accDescr and use braces:

flowchart TD
    accTitle: Account recovery decision flow
    accDescr {
      A user submits an email address. If an account is found, the system sends
      a recovery link and confirms that it was sent. If no account is found,
      the system shows recovery help and offers a support contact.
    }
    A[Submit email address] --> B{Account found?}
    B -->|Yes| C[Send recovery link]
    B -->|No| D[Show recovery help]

Mermaid does not define a universal 100-character title limit or 500- character description limit — write the shortest title that distinguishes the diagram on the page, and keep the description concise enough to be useful as an image description (move detailed steps into visible structured HTML instead of stuffing them into accDescr).

Do not invent directives like %%a11y-node or %%a11y-edge — these are not standard Mermaid syntax; they're comments unless a project-specific preprocessor implements them, and should never be documented as Mermaid features.

Parse and render the source with the project's exact Mermaid version to catch syntax-support and diagram-type differences — accTitle/accDescr support varies by version and diagram type.

Writing Useful Titles and Descriptions

Title: identifies the subject, distinguishes it from others on the page, includes the diagram type only when it helps, avoids filenames/internal IDs/ generic labels ("Diagram"). Examples: "Account recovery decision flow", "Payment service request sequence", "Order states and permitted transitions".

Description: states the purpose, starting context, main elements/ participants, essential sequence/hierarchy/relationships, important decisions/exceptions/outcomes, and where a complete alternative is available. Do not merely list colors and shapes ("A blue rectangle points to a green diamond" describes appearance, not meaning) and do not flatten every node label into one long sentence — that's hard to navigate and loses structure.

Also provide a visible title/description — a page heading or figure caption helps everyone, not just AT users:

<figure aria-labelledby="recovery-flow-heading">
  <h2 id="recovery-flow-heading">Account recovery flow</h2>
  <pre class="mermaid">
flowchart TD
    accTitle: Account recovery decision flow
    accDescr: A recovery request either sends a link or offers additional help.
    A[Submit email address] --&gt; B{Account found?}
  </pre>
  <figcaption><a href="#recovery-flow-description">Read the account recovery steps</a>.</figcaption>
</figure>

(The > escaping is only needed when Mermaid source is embedded directly in HTML — Markdown Mermaid fences don't need it.)

Serious: Provide a Visible Structured Alternative for Complex Diagrams

accDescr creates an SVG description, not a navigable document structure. Complex diagrams need visible HTML with headings, lists, tables, and links.

<section id="recovery-flow-description" aria-labelledby="recovery-flow-description-heading">
  <h3 id="recovery-flow-description-heading">Account recovery steps</h3>
  <ol>
    <li>The user submits an email address.</li>
    <li>The system checks for an account:
      <ul>
        <li>If found, send a recovery link and confirm it was sent.</li>
        <li>If not found, show recovery help and offer support.</li>
      </ul>
    </li>
  </ol>
</section>

Keep alternatives synchronized — generate the diagram and its alternative from the same reviewed data model when possible; if maintained separately, require both in the same change, compare nodes/values/edges, and assign an owner for content review. Do not expose raw Mermaid source as the only alternative — it may help developers but is not an equivalent explanation for all users.

Serious: Understand Mermaid's Generated SVG (Do Not Hand-Author a Different Pattern)

Current Mermaid documentation states the renderer automatically adds aria-roledescription based on diagram type, and when accTitle/accDescr are provided, generates <title>/<desc> elements with aria-labelledby referencing the title and aria-describedby referencing the description — separately, not both combined into one aria-labelledby:

<svg aria-labelledby="generated-title-id"
     aria-describedby="generated-description-id"
     aria-roledescription="flowchart-v2"
     id="generated-diagram-id">
  <title id="generated-title-id">Account recovery decision flow</title>
  <desc id="generated-description-id">A recovery request either sends a link or offers additional help.</desc>
</svg>

Do not rewrite both as one aria-labelledby value without a tested reason — a label identifies the image; a description provides additional information; they're distinct relationships. Exact IDs/classes/attributes can change by Mermaid version — don't copy generated IDs into source or depend on undocumented internal class names.

Node-by-node semantics are not automatic. accTitle/accDescr describe the diagram as a whole — they do not make every node/edge/arrow a useful accessible object. Do not automatically add role="list"/role="listitem" to generated SVG groups — a visual layout is not necessarily a list, DOM order may not match reading order, and list semantics can't express branching, cardinality, or graph relationships. If users need to inspect individual items, build and test a purpose-specific interactive component — generated SVG internals are a fragile foundation for a complex widget.

IDs need to be unique in the final HTML document (not across unrelated pages). Let Mermaid manage its generated IDs; check for duplicates when several inline diagrams share a page; use deterministicIds config for stable snapshot testing; preserve every referenced ID through sanitization/export.

Serious: Embedding and Export Modes Change the Rules

<figure>
  <img src="account-recovery-flow.svg"
       alt="Account recovery either sends a reset link or provides additional support."
       width="960" height="540">
  <figcaption><a href="#recovery-flow-description">Read the complete account recovery steps</a>.</figcaption>
</figure>

Do not leave HTML alt empty just because the source SVG contains <title>/<desc>.

Choosing Clear Diagram Content

Nodes/participants: human-readable labels, not internal IDs; expand uncommon abbreviations; keep labels concise but meaningful; give visually similar nodes distinct textual labels; don't use shape alone to distinguish a decision/process/database/external system.

Edges/relationships: label branches when the outcome isn't otherwise clear — "Approved"/"Needs revision" over generic "Yes"/"No"; preserve direction and source→destination meaning in the alternative; explain unlabeled relationships depending on line style or position; don't use connector color alone to encode status or relationship type.

Reading order/layout: choose a direction matching content and document language; reduce line crossings; avoid layouts implying relationships through proximity alone. Visual placement is not a programmatic reading order — state the intended sequence/hierarchy in the description and structured alternative.

Moderate: Color and Contrast

Apply WCAG 2.2 ratios: normal text ≥4.5:1; large-scale text ≥3:1 (don't treat 18 CSS pixels alone as the large-text threshold — check WCAG's actual definition); visual information required to understand meaningful nodes/ boundaries/connectors/states ≥3:1 (WCAG 1.4.11). Not every decorative fill needs 3:1 against every neighboring fill when labels/outlines preserve the information. Use WCAG 2.x contrast for WCAG 2.2 conformance — APCA/WCAG 3 work is research to monitor, not a substitute test.

Combine color with: direct labels, line styles, patterns, icons with text alternatives, border treatments, or values in the structured alternative — for status, ownership, participant groups, critical paths, and selected states.

Test actual themes — centralize theme configuration rather than styling individual elements by unstable internal selectors; test text/fills/borders/ connector lines/arrowheads/labels/focus/data marks in every supported light and dark presentation and inside the actual background/container colors; test forced-colors mode and keep the structured alternative usable when SVG styling is lost. Do not assume selecting Mermaid's dark theme or adding a prefers-color-scheme rule automatically produces accessible dark mode — recheck themes after Mermaid updates.

Responsive Layout, Zoom, and Reflow

Include a useful viewBox in exported SVG; don't clip the diagram at 200%/ 400% zoom; let users open a larger view or download an SVG when that helps; keep the visible title/summary/alternative/controls reflowable; test long labels and translations; never disable browser zoom. WCAG 1.4.10's two-dimensional-layout exception can apply to a complex diagram itself, but the surrounding title, description, controls, and structured alternative still need to reflow.

Serious: Keyboard and Interactive Diagrams

Static diagrams do not need tabindex="0" — keyboard focus should move to links/buttons/controls, not every decorative SVG group. If a diagram is visually scrollable, ensure keyboard users can reach and scroll its container without becoming trapped, and still provide a structured alternative that doesn't depend on 2D scrolling.

Avoid putting the only path to essential links/actions inside a diagram — provide visible HTML links/controls nearby. If Mermaid links/click actions are enabled: each link is keyboard reachable; purpose is understandable from its accessible name; focus is visible and unobscured; pointer and keyboard actions produce the same result; the accessible name includes the visible label; target size and hover/focus content meet applicable criteria; every action has an equivalent HTML path. Do not simulate a button with a non-focusable generated SVG group.

For interactive exploration (select nodes, expand branches, filter, inspect details), build an accessible interaction model outside the generated static SVG, or use a tested component — Mermaid source plus ARIA attributes does not by itself define an accessible graph-navigation widget.

Motion and Animation

Avoid decorative animation. If animation/auto-updates/moving paths are added by a host or plugin: provide pause/stop/hide controls where WCAG 2.2.2 applies; respect prefers-reduced-motion; avoid flashes exceeding WCAG thresholds; don't use motion as the only change indicator; keep the final state available in text. Don't make accessibility claims about Mermaid animation without testing the exact version/integration.

Critical: Secure and Stable Rendering

Accessibility and security can fail together when untrusted diagram source can inject markup. Keep Mermaid's securityLevel: 'strict' default unless a reviewed use case requires another mode — do not lower security merely to add essential links (put them in HTML instead). Treat user-supplied Mermaid source as untrusted input; use supported sanitization and a restrictive CSP; pin Mermaid/renderer dependencies via the project's lockfile when self-managed; for platform-managed rendering, record the observed version, date, discovery method, and tested capabilities. Do not load production dependencies from an unpinned @latest URL. Limit source size/complexity to prevent rendering failures; render errors as accessible text without exposing sensitive details.

Controlled vs. platform-managed renderers:

GitHub.com Markdown, GitHub Enterprise Server, GitHub Pages, local previews, and exported files are separate rendering surfaces — do not infer support on one surface from successful rendering on another. GitHub Pages does not automatically inherit GitHub.com's Mermaid renderer; the Jekyll theme/plugin/build pipeline determines whether and how Mermaid renders there.

Discover a hosted version with the info diagram:

info

This is a live diagnostic, not a pinned dependency — it may change without a commit, and only reports the renderer on the surface where it's displayed. Record it as a dated plain-text observation, not the only accessible record: Last manually observed on GitHub.com: Mermaid x.y.z on YYYY-MM-DD.

Verify capabilities per surface, not just the version number: required diagram types parse and render; accTitle/accDescr produce the expected title/description/ARIA relationships; the host's sanitizer preserves required semantics; light/dark/forced-colors remain understandable; multiple diagrams per page don't produce conflicting IDs; the structured alternative remains available if rendering fails.

Authoring and Review Workflow

Before authoring: decide whether a diagram materially improves understanding; identify purpose/audience/essential relationships/alternative format; confirm the target renderer supports the diagram type and accessibility syntax.

During authoring: add valid accTitle:/accDescr: lines (no %% prefix); use clear labels; keep visual complexity proportionate; avoid color-only/shape-only meaning; write the structured alternative alongside the source; keep essential links/controls in HTML.

Before publication: render with the pinned production version (or record the platform-managed version and verify capabilities); inspect the generated SVG and final accessibility tree; validate the structured alternative against the diagram; test all supported themes/zoom/viewports/exports; test the actual host (not just an editor preview); record reviewer/version/date/limitations.

AI-generated diagrams: treat as a draft — verify every node/edge/value/ relationship against source material; rewrite generic accessibility descriptions; check the structured alternative matches the final diagram; reject invented Mermaid directives; don't infer accessibility/security/ conformance from successful visual rendering alone.

Linting and Automated Validation

Source checks: the exact source parses with the target renderer; accTitle: is present and not commented out (i.e., not prefixed with %%); accDescr: or accDescr { ... } is present and not commented out; values are non-empty; prohibited custom directives aren't mistaken for real syntax; a complex diagram references a visible structured alternative. Don't enforce arbitrary universal character-count limits.

Rendered-output checks: generated <title>/<desc> exist; aria-labelledby resolves to the title; aria-describedby resolves to the description; IDs unique in the final page; sanitization/optimization didn't remove referenced elements; no unintended focusable descendants; interactive elements have names/roles/states/keyboard behavior/visible focus; render errors exposed as text.

Contrast checks need actual rendered pairs (text vs. effective background, boundaries, connector lines, focus states) in every supported theme — "monochrome" is not a reason to skip contrast testing.

Automation cannot determine whether the description is equivalent, reading order is meaningful, or the diagram is understandable — manual and user review required.

Testing

• Content/equivalent-purpose: state the question the diagram should

answer; identify every essential relationship/branch/value; answer using the visual diagram, then using only the title/description/structured alternative; compare available information and conclusions; correct both representations together

• Keyboard: confirm static SVG isn't an unnecessary Tab stop; operate

every diagram link/control/popup; verify visible focus and logical order; enter/leave scrollable diagrams without a trap; confirm every action has an equivalent HTML path

• Screen reader: confirm the computed accessible name/description; check

whether the diagram is exposed as image/graphic/document by the final host; read the structured alternative by headings/lists/tables; confirm no duplicate/excessive announcements

• Visual/low-vision: normal and large text; 200%/400% zoom; narrow

viewports/orientations; every light/dark presentation; forced-colors mode; confirm labels/nodes/lines/arrowheads/legends stay visible; long translations don't overlap

• Export/platform: test the original Markdown/source preview AND the

production page after sanitization/optimization; record whether each tested renderer is pinned or platform-managed; test inline SVG, external SVG, and raster variants; test print/PDF; test with JavaScript unavailable or rendering failed; test several diagrams on one page for duplicate IDs; retest after Mermaid/plugin/theme/host updates

Common Failures

Definition of Done Checklist

• accTitle: present using correct syntax (no %% prefix), concise and unique
• accDescr: or accDescr { ... } present, explains purpose and key relationships
• Mermaid version confirmed/recorded for the actual publishing surface(s)
• Generated SVG verified: <title>, <desc>, aria-labelledby (title) and

aria-describedby (description) kept as separate relationships

• Complex diagram has a visible structured alternative matched to its diagram type
• Diagram and alternative are synchronized from the same data/change
• All IDs unique within the final page
• Decorative elements excluded from the accessibility tree; no fabricated role="list" on generated groups
• Named edges include contextual labels ("Yes, proceed to X" not "Yes")
• Contrast verified in light, dark, and forced-colors modes
• Static diagrams are not unnecessary Tab stops; essential links exist as real HTML links too
• Each publishing surface (GitHub.com, GitHub Pages, local preview, exports) tested separately
• securityLevel: 'strict' retained unless a reviewed exception is documented
• Tested with a screen reader

Key WCAG Criteria

• 1.1.1 Non-text Content (A) — Critical if no title/alternative
• 1.3.1 Info and Relationships (A)
• 1.4.1 Use of Color (A)
• 1.4.3 Contrast Minimum (AA)
• 1.4.10 Reflow (AA)
• 1.4.11 Non-text Contrast (AA)
• 2.1.1 Keyboard (A)
• 2.4.7 Focus Visible (AA)
• 4.1.2 Name, Role, Value (A)
• 4.1.3 Status Messages (AA)

References

• Full best practices guide
• Mermaid diagram types reference
• Mermaid transformation best practices
• Mermaid: Accessibility Options
• Mermaid: Security
• GitHub: Checking your version of Mermaid
• WAI-ARIA 1.2: aria-roledescription

Standards horizon: These rules target WCAG 2.2 AA.
Monitor: <https://www.w3.org/TR/wcag-3.0/>;