Builder Contract

Root #stage (data-composition-id) needs position: relative; width: <W>px; height: <H>px. Without a resolved height, flex children collapse to ~0 and content piles into the top-left. Automated gates may miss it, so inspect proof snapshots.

Layout before animation

1. Identify the hero frame (the moment most elements are visible) → build THAT in static CSS first, no GSAP.
2. .scene-content fills the scene with padding, not offsets:

   cssCopy

   display: flex;
   flex-direction: column;
   justify-content: center;
   width: 100%;
   height: 100%;
   padding: 120px 160px;
   gap: 24px;
   box-sizing: border-box;

   Never position:absolute; top:Npx on a content container (it overflows). Reserve absolute for decoratives. Keep ≥80px padding (title-safe margin).
3. Entrances: use gsap.from() only for a non-clip element active from t=0. Inside .clip, in sub-compositions, and for later entrances, use explicit fromTo(). The CSS position is ground truth; the tween is the journey to it.
4. Exits: only the final scene animates elements out; between scenes the transition IS the exit.

Timeline / clip contract

• ONE gsap.timeline({paused:true}) on window.__timelines["<id>"]; tl.seek(0); never tl.play().
• Timed elements: class="clip" + data-start/data-duration/data-track-index + a stable id. Timeline-driven groups inside one full-duration clip don’t each need timing attrs.
• Deterministic only — no Date.now() / Math.random() / network. Count-ups tween a proxy object via onUpdate (seek-safe), never a wall-clock counter.

Correctness

• Seek-safe reveal of delayed elements: on a non-clip element or wrapper inside a clip, use one registered timeline fromTo() with an explicit { autoAlpha: 0 } start and { autoAlpha: 1, ... } end. Do not page-load gsap.set() a later .clip, and never target .clip visibility; the framework owns its lifecycle. (Eval finding.)
• Count-ups tween a proxy via onUpdate; they only render when the host advances the timeline with events enabled (tl.time() / non-suppressed seek). A bare seek(t, true) freezes them at 0 — the HF render host must seek with events on. (Eval finding.)
• Clamp at tween bounds; don’t let a spring overshoot past a held value.
• Allowed eases: power1–4, back, bounce, circ, elastic, expo, sine (.in/.out/.inOut).
• One motif per scene. Run hyperframes check; mark intentional overflow data-layout-allow-overflow="true".
• Palette discipline: define all colors in one palette object / CSS custom properties — no inline hex scattered through the markup (for asset-fusion, eyedropper the palette from the asset).