SPEC-105 §5–§6: the timing trigger and the no-JS/SSR safety. A tiny IntersectionObserver behaviour flips data-in-view; enhancement gating guarantees the static page is always complete. JS = when, CSS = how.
reveal behaviour — joins tabs/accordion/datatable/form in packages/behaviors/src/. Observes each [data-reveal]:not([data-reveal="none"]) container; on first intersection it sets data-in-view and unobserves. Theme-agnostic: one state attribute, nothing else.
Reduced-motion path — honour matchMedia('(prefers-reduced-motion: reduce)'): mark everything in-view immediately (belt-and-braces with the global reset).
Enhancement gating (cardinal rule) — SSR/no-JS/crawler render the fully visible final state; never bake opacity:0 that only JS removes. The pre-entrance state is scoped under a root data-animate flag the behaviour adds on boot; no flag → every section fully rendered.
Acceptance Criteria
An IntersectionObserverreveal behaviour sets data-in-view on first intersection and unobserves; under prefers-reduced-motion it marks all in-view immediately.
Enhancement gating: SSR/no-JS/crawler render the fully visible final state (no opacity:0 baked in); the pre-entrance state is scoped under a root data-animate flag the behaviour adds on boot; the WORK-352 reduced-motion reset still neutralises motion.
The behaviour is theme-agnostic (sets only data-in-view/the root flag) and frame-agnostic; tests cover the observe→reveal→unobserve cycle, the reduced-motion path, and the no-JS/SSR complete-render guarantee.
Dependencies
WORK-431 — observes data-reveal. Pairs with WORK-432 (the gated pre-entrance CSS).