Determinism Rules

Animation Runtime Contract

GSAP is the primary runtime. The core requirement is generic: animation state must be seekable from HyperFrames time.

For GSAP:

• Use gsap.timeline({ paused: true }).
• Register it on window.__timelines["<composition-id>"], keyed by the composition root’s data-composition-id. You do not need to write window.__timelines = window.__timelines || {} first: the runtime creates the registry before your inline scripts evaluate.
• Building inside an async callback is supported. document.fonts.ready(...) and friends are the documented setup path. What you must not do is register the key before the build finishes. An empty timeline registered early is treated as ready and nested empty, so the animation renders blank (lint: gsap_timeline_registered_before_async_build, error). Assign window.__timelines[id] = tl at the end of the callback, after the tweens are added, and optionally call window.__hfForceTimelineRebind() right after.
• If the key does not match the root’s data-composition-id, the runtime still binds it when it is the only registered timeline. With two or more registered, a mismatched key leaves the render frozen at t=0.
• Do not call tl.play() for render-critical motion.
• Do not create empty tweens only to set duration; use data-duration on the clip instead.

Use the hyperframes-animation skill for tween syntax, position parameters, eases, and performance rules. Non-GSAP duration inference lives in hyperframes-animation/adapters/.

Determinism Rules

Rendered frames must be reproducible from the requested time. Do not use any of the following for visual state:

• Date.now(), performance.now(), or any render-time clock.
• Unseeded Math.random(). Use a seeded PRNG if random-looking placement is needed.
• Render-time network fetches for required assets. Inline or pre-bundle them.
• Hover, scroll, pointer, or focus state. The renderer has no input events.
• Infinite loops such as repeat: -1. Compute a finite count: repeat: Math.max(0, Math.floor(duration / cycleDuration) - 1) — floor, not ceil (ceil overshoots data-duration and trips the gsap_repeat_ceil_overshoot lint; max(0, …) avoids a negative repeat = infinite).

Also avoid:

• Tweening display, raw visibility, or autoAlpha on a clip element: HyperFrames timing owns a clip’s visibility, and lint rejects it (gsap_animates_clip_element). Fade with opacity, or tween a child wrapper. Do not tween class="clip".
• There is no fixed allowlist of animatable properties. lint enforces a denylist, so filter, clipPath, strokeDashoffset, width, height and similar are all legitimate targets. Prefer transforms and opacity where you have the choice, for performance rather than correctness. The per-runtime detail lives in hyperframes-animation/adapters/.
• Animating the same property on the same element from multiple timelines at the same time — GSAP’s overwrite behavior is order-dependent and can flip between renders.

Layout Contract

Build the visible end-state in static HTML and CSS first, then animate from/to that state.

• The composition root has fixed pixel frame dimensions.
• The root composition’s total duration (render length / frame count) is fixed at compile time, read once from the static root data-duration before scripts run, like data-width / data-height. A script or --variables value that rewrites the root data-duration afterward is ignored. To vary render length per output, author the root data-duration directly. (A clip’s own data-duration is re-read from the live DOM, so scripts/variables can still drive clip lengths. Only when the root omits data-duration does the renderer probe the live DOM / timeline for total length.)
• Scene containers should fill the scene with width: 100%; height: 100%; box-sizing: border-box.
• Use padding, flex, grid, and max-width for layout. Avoid positioning main content with hardcoded top/left offsets when a layout container can do it.
• Use position: absolute for layers and decorative elements, not as the default content-layout strategy.
• Prefer transforms and opacity for animation.
• Keep text inside its intended container. For dynamic text, use max-width, wrapping, or window.__hyperframes.fitTextFontSize(text, { maxWidth, fontFamily, fontWeight }).
• For text measurement without DOM reflow, use window.__hyperframes.pretext. Measure off a canvas instead of writing into the page and reading it back, so nothing reflows: pretext.prepare(text, font) then pretext.layout(prepared, maxWidth, lineHeight) → { lineCount, height }. prepare does the font measurement; everything downstream of a prepared string is arithmetic and cheap enough to run per frame. fitTextFontSize is built on it.
  • layout gives you height, not width. To size a container to its text (shrinkwrap), use pretext.prepareWithSegments(text, font) and then pretext.measureNaturalWidth(prepared) for the single-line width, or pretext.measureLineStats(prepared, maxWidth) for { lineCount, maxLineWidth }.
  • font is a CSS font shorthand string, e.g. "700 90px Inter".
  • clearCache and setLocale are deliberately not exposed: they mutate state shared across compositions, which would make a render depend on what ran before it.
• Do not use <br> in body text. Forced breaks ignore the actual rendered font width and produce an extra break when the line already wraps naturally, causing overlap. Let text wrap via max-width. Exception: short display titles where each word is deliberately on its own line.
• Transformed elements must be block-level + sized. transform/scaleX/scaleY is a no-op on an inline <span>, and scaling an auto-width (0px) element shows nothing → invisible bars/fills. Give them display: block/inline-block/flex-item and a real width/height (e.g. width: 100% inside a sized parent). (Silent — automated gates may miss it.)
• Absolutely-positioned decoratives that pulse or overshoot (yoyo scale, back.out) need clearance at their peak size and must not straddle an overflow: hidden edge — else they overlap a neighbor or get clipped. Position for the largest frame, not the resting one. (silent.)