[scanner] html scroll-behavior: smooth ignores prefers-reduced-motion #280

What

style.css:92 sets html { scroll-behavior: smooth; } unconditionally. Under prefers-reduced-motion: reduce the rest of the site switches animation off — style.css:274, :507, :578 zero the transitions/animations, and the projects-carousel script picks behavior: 'auto' for track.scrollTo when reduceMotion.matches (index.html:1046, :1077) — but the document itself keeps animating every in-page jump: the skip link (index.html:33 → #main), the header nav anchors, the hero #slide-… hash navigation and Back to top-style links all smooth-scroll for a user who asked for no motion.

Why it matters

WCAG 2.3.3 (Animation from Interactions) / the reduce-motion contract the carousel code already honours. For vestibular users a full-page smooth scroll is the largest motion the page produces, and it is the one piece that ignores the preference.

Fix

Scope the rule to users who have not opted out:

@media (prefers-reduced-motion: no-preference) {
  html { scroll-behavior: smooth; }
}

(or equivalently add html { scroll-behavior: auto; } to an existing prefers-reduced-motion: reduce block). Consider a style-contract.test.mjs case asserting scroll-behavior: smooth only appears inside a no-preference media query so it cannot regress.

Evidence

• style.css:92 — html { scroll-behavior: smooth; -webkit-text-size-adjust: 100%; } at top level, no media query
• style.css:274-277, :507-512, :578-581 — existing (prefers-reduced-motion: reduce) blocks, none touches scroll-behavior
• index.html:1046 — function scrollBehavior() { return reduceMotion.matches ? 'auto' : 'smooth'; } (script already honours the preference)
• No scroll-behavior / reduced-motion assertion in scripts/style-contract.test.mjs

— hive: agent=scanner backend=copilot model=claude-fable-5.1 copilot=1.0.88