Accessibility is a coding standard, not a checklist

Accessibility is authored into the widget as you write it — the same tier as "the button actually works." There is no lint for any of this: flutter_lints and very_good_analysis ship zero a11y rules. Enforcement is the widget itself, review of your own diff, and widget tests. Applies to every interactive View, every Icon/Image, every state, and every color choice.

Testing mechanics (guideline matchers, golden lanes, RTL, honest conformance limits) live in widget-golden-and-a11y-testing; this skill is the authoring discipline.

Non-negotiable rules

1. Every interactive node gets Semantics(button: true, label: ...) — a raw GestureDetector with no semantics silently locks out every screen-reader and switch user. Correct semantics make TalkBack, VoiceOver, Switch Access and Switch Control work for free; it is definition-of-done, never a backlog ticket.
2. Every Icon/Image gets a semanticLabel, or is wrapped in ExcludeSemantics because it is decorative. There is no third option — an unlabelled Icon is invisible to a screen reader.
3. Read a11y state from MediaQuery at build time, never from app state. MediaQuery is an InheritedWidget with correct-by-construction invalidation; pushing it through a provider trades a compiler-guaranteed rebuild for a frame of staleness in the one area where being wrong is total failure.
4. Never MediaQuery.withClampedTextScaling, never textScaleFactor. Both are clamping hacks that silently defeat the text-scale matrix while contrast and tap-target guidelines still pass green. Enforce with a source grep (see Anti-patterns).
5. Never FittedBox, computed fontSize, or TextOverflow.ellipsis to make a label fit. They turn "doesn't fit at 200%" from a loud test failure into truncated or shrunk text on a device. Build flexible heights and let text wrap.
6. Never encode state through color alone. Pair every stateful signal with at least one non-color channel — icon + text label + shape/pattern + position. Color-only state is invisible under invertColors, grayscale color-correction, and to every screen-reader user.
7. Contrast is measured against the composited background in both themes — body/icon text ≥ 4.5:1 (AA 1.4.3), large/hero text ≥ 3:1 (AA) — large text never has to clear a higher bar than body. Never test against the nominal token color when a fill, gradient, or translucency sits between text and surface.
8. Every interactive target is ≥ 44×44 logical pixels and single-tap. No long-press or precise-gesture-only affordances; hand-off and motor-impaired use demand a large forgiving target.
9. Traversal order is authored with sortKey, never inherited from layout — visual position optimized for the thumb actively pessimises linear screen-reader/switch scanning.
10. Honor boldText; keep haptics through reduced motion. Hardcoding fontWeight throws the setting away. Read disableAnimations from MediaQuery here, but resolve how motion collapses through design-system-structure's reduced-motion token + resolveMotion helper (that skill owns the mechanism) — an animation that still conveys meaning with no reduced-motion path is a bug.