A11y LLM Evaluation Harness and Dataset is a test suite for measuring how well language models generate accessible HTML. It runs agent-generated pages in a browser and evaluates them with automated accessibility checks.
Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx skills add microsoft/a11y-llm-eval --skill building-accessible-uigit clone --depth 1 https://github.com/microsoft/a11y-llm-evalWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/microsoft/a11y-llm-eval/building-accessible-ui)<a href="https://agentmods.dev/skills/microsoft/a11y-llm-eval/building-accessible-ui"><img src="https://agentmods.dev/badge/skills/microsoft/a11y-llm-eval/building-accessible-ui/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/microsoft/a11y-llm-eval/building-accessible-ui"><img src="https://agentmods.dev/badge/skills/microsoft/a11y-llm-eval/building-accessible-ui.svg" alt="Reviewed on agentmods" width="80" height="20"></a>What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00112 | $0.02435 |
| Opus 5.5 | $0.00045 | $0.00974 |
| Sonnet 5.5 | $0.00022 | $0.00487 |
| Haiku 4.5 | $0.00011 | $0.00244 |
Grade A, and why
building-accessible-ui scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 2mo ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
How it starts
The opening of the file, as written. The whole thing — 101 lines — stays where its author put it; the contents beside it link to each section on GitHub.
building-accessible-ui
Checklist for producing and reviewing accessible UIs. Each rule leads with the platform-agnostic principle and, where relevant, the Web (HTML + ARIA + CSS) implementation. Apply the web guidance only when the output is web.
Detailed rationale lives in references/; widget-specific guidance in components/. Open a file only when it's relevant to the current task. Do not preload. Every file opened and every line a tool prints stays in context — don't re-read.
Accessibility constitution
Ground rules. Use them to resolve conflicts and decide how much custom work is justified. The checklist below is their mechanical application.
1. Accessibility is a core outcome
A UI inaccessible to realistic users is not "done". Treat accessibility as a first-class criterion alongside correctness, performance, and security — not a finishing step. When scope must be cut, record the gap explicitly. Never claim output is "fully accessible"; state what was addressed and known limitations.
2. Build for real people
Evaluate designs against these personas; if a decision breaks one, justify it and offer an alternative:
- Screen reader — landmarks, headings, accessible name/role/state, reading order.
- Keyboard-only — Tab/arrows/Enter/Space/Escape, visible focus, no traps.
- Low-vision — zoom, reflow, contrast, Forced Colors.
- Cognitive — plain language, clear labels, actionable errors, forgiving interactions.
- Deaf / hard of hearing — captions/transcripts; no sound-only cues.
- Motor / voice / switch — large hit targets, named controls, no precise/timed gestures.
- Situational — sunlight, one-handed, noisy, flaky network.
3. Implementation priority
Use the highest option that fits:
- Existing accessible component in this codebase / design system.
- A component library.
- Native platform semantics (
<button>,<a href>,<input>,<label>,<fieldset>/<legend>,<dialog>,<details>,<nav>,<main>, headings). - Native element + minimum necessary ARIA (
aria-describedby,aria-expanded,aria-current, etc.). - Fully custom ARIA widget — only when nothing above fits, and only if you implement the APG keyboard, focus, and state behavior end-to-end.
What ships with it
21 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.
- AUTHORING.md 5.1 KB
- components/checkbox-group.md 4.4 KB
- components/disclosure-widget.md 3.4 KB
- components/forms.md 3.7 KB
- components/modal-dialog.md 6.7 KB
- components/page-layout.md 4.3 KB
- components/radio-button-group.md 4.4 KB
- components/README.md 762 B
- components/single-checkbox.md 2.2 KB
- references/contrast-forced-colors.md 2.9 KB
- references/images-graphics.md 2.4 KB
- references/keyboard-focus.md 3.9 KB
- references/navigation.md 2.2 KB
- references/reflow.md 2.1 KB
- references/specs-documentation.example.md 6.3 KB
- references/specs-documentation.md 5.9 KB
- references/specs-documentation.template.md 3.5 KB
- references/status-messages.md 4.3 KB
- references/structure.md 2.3 KB
- references/tables-grids.md 2.7 KB
- references/testing.md 5.7 KB
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 2mo ago First seen · 101 lines · 112 tokens per session scan A 2d8fff1426dc
building-accessible-ui is a skill published in the GitHub repository microsoft/a11y-llm-eval (62 stars, last pushed 5mo ago), licensed MIT. It adds 112 tokens to every session and 2,435 once invoked, about $0.0004 per session on Opus 5.5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.
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