Docs: add A11y page to coding guidelines

.cspellignore

@@ -64,4 +64,5 @@ timestamptz
 formatjs
 autoplay
 authproxy
-xact
+xact
+WCAG

docs/docs/9-coding-guidelines/13-accessibility.md

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+---
+title: Accessibility
+---
+
+Comet projects aim to conform to [WCAG 2.1 Level AA](https://www.w3.org/TR/WCAG21/).
+
+## POUR Principles
+
+- **Perceivable:** Provide text alternatives, captions, and sufficient contrast.
+- **Operable:** All functionality must be keyboard accessible.
+- **Understandable:** Use predictable behavior, clear instructions, and helpful error messages.
+- **Robust:** Ensure content works with a wide range of devices and assistive technologies.
+
+## Key Rules
+
+- Use semantic HTML (`<nav>`, `<main>`, `<button>`, `<label>`, etc.).
+- Color contrast: at least **4.5:1** for normal text, **3:1** for large text.
+- All interactive elements must be keyboard operable with visible focus indicators.
+- Images need meaningful `alt` text; use `alt=""` for decorative images.
+- Associate form inputs with `<label>` (or `aria-label` / `aria-labelledby`).
+- Respect `prefers-reduced-motion`; avoid autoplay animations for users who have requested reduced motion.