a11y-render-gate

An accessibility gate inside the agent loop.

An agent writing UI has no perception. It emits text-gray-400 on white, a <div onClick>, an 18px icon button, and a CSS reset containing outline: none with no replacement. Then it says "done." It cannot see the result, so it cannot know. The human reviewing it usually can't either — nobody eyeballs a contrast ratio.

a11y-render-gate renders the UI the agent just wrote, measures it, and hands back computed facts with fixes attached — then blocks the turn until they're fixed.

a11y-render-gate FAIL  http://localhost:5173/checkout
desktop 1440x900, light · 47 elements · 1.2s

critical focus-visible  ×4
  button "Continue"  button.btn-primary
    0 pixels change when focused — no indicator renders
    → outline is set to none with no replacement — add a :focus-visible ring.
       :focus-visible {
         outline: 2px solid #1a5fd0;
         outline-offset: 2px;
       }
    same fix applies to: input#email, input#card, button "Back"

serious contrast
  p "Secondary text"  p.muted
    #8a8a8a on #ffffff = 3.45:1, need 4.5 (14px)
    → Set color to #767676 (4.54:1 against #ffffff).
       color: #767676;

Why this is not another linter

All of them are built so a human reads a report afterwards. None make the agent check itself and fix it in the same turn.

Two things make this different:

1. Findings are shaped like fixes.

axe:        color-contrast: Elements must have sufficient color contrast (serious)
            <button class="btn-secondary">Continue</button>

a11y-render-gate:  #8a8a8a on #ffffff = 3.45:1, need 4.5:1 (14px, normal)
            → Set color to #767676 (4.54:1 against #ffffff)

The second one an agent acts on in one step. The first requires a guess.

2. Focus visibility is measured, not inferred.

Screenshot the element at rest → focus it → screenshot again → diff the crops. Zero changed pixels means the focus indicator does not exist on screen. This isn't statically expressible, axe doesn't attempt it, and it's epidemic in generated UI because every CSS reset kills the default outline.

Related MCP server: axe-devtools-mcp

Install

npm install -D a11y-render-gate
npx playwright install chromium    # ~150MB, one time
npx a11y-render-gate doctor        # check everything is wired up

Requires Node 20+. Chromium is the only browser used.

As a Claude Code plugin

Bundles the MCP server, the blocking Stop hook, and the skill in one unit:

claude --plugin-dir /path/to/a11y-render-gate

The plugin runs dist/, so it needs a build first — npm install in the a11y-render-gate directory is enough.

Use

a11y-render-gate check http://localhost:5173/checkout   # a running app
a11y-render-gate check ./component.html                 # a standalone file
a11y-render-gate check --story ui-button--secondary     # a Storybook story
a11y-render-gate check --component src/ui/Button.tsx    # one component, in isolation
a11y-render-gate check                                  # every route in your config
a11y-render-gate check / --within '.checkout-form'      # just one subtree
a11y-render-gate doctor                                 # what can it currently reach?

Exit codes: 0 pass, 1 findings at or above your threshold, 2 couldn't check (so CI can tell "broken" apart from "inaccessible").

As an MCP tool

State behind an interaction is reachable:

a11y_check({ url: "/cart", actions: [{ click: ".open-cart" }, { wait: 300 }] })

Checking one component in isolation

No dev server, no Storybook needed — but it does not try to reconstruct your build. It loads your Vite, from your node_modules, pointed at your config, so aliases, plugins, PostCSS and Tailwind all apply exactly as they do in the app.

a11y-render-gate check --component src/ui/Button.tsx --export Button
a11y-render-gate check --component src/ui/Card.tsx --props '{"title":"Hello"}'

React, Vue, Svelte and plain modules are supported; the framework comes from your package.json.

Components that need a router, theme or store need a wrapper, because providers cannot be inferred from source and guessing produces confident nonsense:

// a11y.wrapper.tsx
export default function Wrapper({ children }) {
  return <ThemeProvider><Router>{children}</Router></ThemeProvider>;
}

a11y-render-gate check --component src/ui/Nav.tsx --wrapper ./a11y.wrapper.tsx

A component that throws or renders nothing is reported as an error, never as a pass. An empty page has no accessibility defects, so silently certifying a broken component as accessible would be the worst thing this tool could do.

Requires vite in the project. Next.js has no stable programmatic dev server — use url or storybook there.

What it checks

Where the value is concentrated

• Broken wiring that looks correct. <label for="emial"> names nothing. aria-labelledby pointing at a removed node names nothing. Both read as correct in review. Reading the browser's accessibility tree is the only way to see it.

• Contrast that isn't what the CSS says. 30%-opacity black text isn't black. Text on a gradient has no single background — axe reports "incomplete", this samples the pixels and returns a number.

• The states nobody checks. A :hover colour is chosen by eye and never measured, so controls go unreadable exactly while you point at them.

Configuration

a11y-render-gate.config.json. Its presence is the opt-in — the Stop hook does nothing in a project without one, so installing the plugin never changes how an unrelated repo behaves.

{
  "level": "AA",
  "sources": {
    "baseUrl": "http://localhost:5173",
    "storybookUrl": "http://localhost:6006",
    "routes": ["/", "/checkout"],
    "component": { "wrapper": "./a11y.wrapper.tsx" }
  },
  "viewports": [
    { "name": "mobile",  "width": 390,  "height": 844 },
    { "name": "desktop", "width": 1440, "height": 900 }
  ],
  "themes": ["light", "dark"],
  "rules": {
    "state-contrast": { "enabled": true },
    "axe": { "enabled": true }
  },
  "ignore": [".third-party-widget"],
  "failOn": ["critical", "serious"],
  "baseline": ".a11y-render-gate/baseline.json"
}

Some production sites reject Playwright's default headless user-agent. Set "userAgent" (or pass --user-agent) when a real page returns 403.

Checking dark is worth the extra pass: contrast defects hide there constantly, because the light palette is the one anybody looks at.

Adopting in an existing codebase

The first run on a real app produces a lot. Draw a line and hold it:

a11y-render-gate check /
a11y-render-gate baseline accept --note "pre-existing, tracked in PROJ-482"

Only new findings fail from now on. Without this step the gate blocks everything on day one and gets disabled permanently, which is the most common way tools like this die.

The baseline records problems, not elements. One grey that fails contrast on 240 elements is one entry, and it keeps matching when the page regenerates with different ids — which is what makes a baseline survive on a real site rather than going stale on the next deploy.

The Stop hook

Five guards run before it is ever willing to block:

1. stop_hook_active           → exit 0   never loops
2. A11Y_RENDER_GATE_DISABLE   → exit 0   escape hatch
3. no config in the project   → exit 0   opt-in only
4. no UI files changed        → exit 0   git-aware
5. dev server unreachable     → exit 0   never block on infrastructure
                                         the user didn't start
→ otherwise: check, and exit 2 with the findings if it fails

That conservatism is deliberate. A gate that blocks for a bad reason once gets uninstalled, and then it prevents nothing at all.

Design notes

One collection pass, pure probes. A single page.evaluate captures every element's geometry, computed styles, background stack, and ancestry. Probes are pure functions over that data — testable with no browser, and one DOM traversal per check instead of one per rule.

Token budget is a design constraint. A raw axe dump is 5–10k tokens; a check that expensive gets called once and then avoided, which defeats the point, since the value is in re-running after a fix. Target is ~1200 tokens: severity ordering, identical fixes collapsed, capped per rule, full JSON written to .a11y-render-gate/last-run.json as the escape valve. Re-runs report movement (2 fixed · 1 still failing · 0 new) rather than restating everything.

False positives are worse than misses. For a gate, a wrong finding costs more than a missed one, because it burns the tool's credibility. The WCAG 2.2 spacing and inline exceptions are implemented; disabled-control contrast is exempt from 1.4.3 and reported as advice, never as a failure; a box-shadow focus ring is recognised as valid. The test suite's most important case is a correct page that must produce zero findings.

Status — v0.3.0

Working and covered by tests: the CLI, the MCP server, the blocking Stop hook, the Claude Code plugin, all seven rules, and all four sources — url, html, storybook and component. 80 tests pass, fixture-driven.

Validated against five real pages, not only fixtures — a 3,000-element Wikipedia article, a news front page, a React documentation site, MDN, and a production homepage. That exercise removed three false-positive classes that every hand-written fixture had missed — cursor: pointer alone is no longer treated as evidence of a control, the WCAG inline exception now measures against the containing block, and visually hidden skip links are no longer reported for contrast.

Not built yet:

• Source mapping — findings carry the offending element's classes, but not a file:line.

• Windows — CI covers Linux; developed on macOS.

• Sites behind bot protection may still refuse to load even with a custom user-agent, if they use a JavaScript challenge.

Interfaces may change before 1.0.

Limitations

Not a full WCAG audit, and no substitute for testing with real assistive technology. It checks what a browser can compute. It does not evaluate whether alt text is good, only whether it exists; it does not cover cognitive accessibility; and screen-reader behaviour differs from the accessibility tree in ways this cannot see.

Automated checks catch roughly a third of WCAG issues. This aims to make that third free and immediate, not to pretend it is all of them.

Development

npm install && npx playwright install chromium
npm test          # fixture-driven, ~5s
npm run build

Fixtures in fixtures/broken/ declare their expected findings in a header comment; fixtures/clean/ must yield zero.

MIT.