Determinism, Animation Runtime, and Layout

HyperFrames seeks compositions frame-by-frame. Every frame must be reproducible from its time value alone: same input time, same pixels. Three contracts enforce this: the animation runtime contract, the determinism rules, and the layout contract.

Animation Runtime Contract

Animation state must be seekable from HyperFrames time. Build runtime state synchronously during page initialization, register it before render validation can sample the page, and never use a free-running clock for render-critical motion.

Generic requirements:

Create timelines, animations, and seek handlers synchronously during page initialization.
Do not build render-critical timelines inside async, Promise, setTimeout, request callbacks, or event handlers. The renderer can sample before they finish.
Keep durations and loop counts finite unless the root has an explicit data-duration.
Do not create empty tweens only to set duration. Use data-duration on the composition root or clip.
Do not call playback methods such as play() for render-critical motion. HyperFrames owns the clock.

Anime.js Default Contract

Anime.js is the first-party default authoring path.

<script src="https://cdn.jsdelivr.net/npm/animejs@4.5.0/dist/bundles/anime.umd.min.js"></script>
<script>
  const tl = anime.createTimeline({ autoplay: false });
  tl.add(".hero", { opacity: [0, 1], duration: 600 }, 0);
  // helper registers id, labels, finite duration; seconds-based seek for the runtime
  hyperframesAnime.register("main", tl, { labels: { intro: 0 } });
</script>

Rules:

Use anime.createTimeline({ autoplay: false }) or anime.animate(..., { autoplay: false }). Always write autoplay: false explicitly.
Register only through hyperframesAnime.register("<composition-id>", instance, { labels }).
The registration id must equal the composition root's data-composition-id.
Labels passed to hyperframesAnime.register are in seconds, matching data-duration and clip timing. Anime.js .add(..., position) values are milliseconds.
Prefer explicit root data-duration. If it is omitted, the anime.js adapter infers render duration from the longest finite registered anime.js instance.
Infinite or unbounded anime.js loops require explicit root data-duration.
anime.stagger(...) is deterministic when its configuration is fixed or seeded. Do not derive stagger amounts, order, or jitter from unseeded Math.random() or wall-clock reads.

ONE-PROPERTY-OWNER rule: two independently registered anime.js instances must never animate the same CSS property on the same element at overlapping times. HyperFrames seeks registered instances independently, and the final owner of a property can become order-dependent. Anime.js has no automatic overwrite manager for this case. If two effects need to touch the same property, put both tweens inside the same registered instance or timeline.

Wrong:

const fade = anime.createTimeline({ autoplay: false });
fade.add("#card", { opacity: [0, 1], duration: 800 }, 0);
hyperframesAnime.register("main", fade, { labels: { intro: 0 } });

const pulse = anime.createTimeline({ autoplay: false });
pulse.add("#card", { opacity: [1, 0.7], duration: 600 }, 300);
hyperframesAnime.register("main", pulse, { labels: { pulse: 0.3 } });

Right:

const tl = anime.createTimeline({ autoplay: false });
tl.add("#card", { opacity: [0, 1], duration: 800 }, 0);
tl.add("#card", { opacity: [1, 0.7], duration: 600 }, 800);
hyperframesAnime.register("main", tl, { labels: { intro: 0, pulse: 0.8 } });

The U3 determinism gate cleared anime.js springs (anime.createSpring), SVG morph (anime.svg.morphTo), SVG drawable/line-draw (anime.svg.createDrawable), split text (anime.text.split), nested .sync() timelines, and seeded stagger. No anime.js features are scoped out of the v1 authoring contract.

GSAP Supported Non-Default Contract

GSAP remains a supported adapter. Use it when a project is already GSAP-authored or when the GSAP-specific API is the right fit.

For GSAP:

Create the timeline synchronously during page initialization.
Use gsap.timeline({ paused: true }).
Register it using the GSAP adapter contract documented in hyperframes-animation/adapters/gsap.md.
The registry key must match data-composition-id on the composition root.
Do not call tl.play() for render-critical motion.
Do not build timelines inside async, Promise, setTimeout, or event handlers.
Do not create empty tweens only to set duration. Use data-duration instead.
Do not gsap.set() clip elements from later scenes. They are not in the DOM at page load. Use tl.set(selector, vars, time) inside the timeline at or after the clip's data-start.
Infinite loops such as repeat: -1 are forbidden. Compute a finite count: repeat: Math.max(0, Math.floor(duration / cycleDuration) - 1). Use floor, not ceil because ceil overshoots data-duration and trips the gsap_repeat_ceil_overshoot lint; max(0, ...) avoids a negative repeat becoming infinite.
Animating the same property on the same element from multiple timelines at the same time is order-dependent and can flip between renders. Keep a single owner for each render-critical property.

Use the hyperframes-animation skill for tween syntax, position parameters, eases, and performance rules.

Duration Contract

The render engine needs a positive total duration before it will capture a single frame. Without one, capture fails outright with "Composition has zero duration."

Runtime inference:

Anime.js: longest finite registered anime.js instance. Infinite or unbounded loops cannot be inferred.
GSAP: registered finite GSAP timeline duration.
CSS: longest animation-delay + animation-duration x finite animation-iteration-count across animated elements (offset by each element's data-start). animation-iteration-count: infinite cannot be inferred.
WAAPI: longest element.animate() effect's getComputedTiming().endTime. Infinite iterations cannot be inferred.
Lottie: the registered animation's native length (totalFrames / frameRate, or the dotLottie player's own duration), always finite regardless of loop.
Three.js: not inferable. The three adapter only forwards time via hf-seek; it has no AnimationClip/AnimationMixer inspection.

data-duration on the root [data-composition-id] element is optional whenever every animation on the page has a finite duration source. It is required when the composition has an infinite/unbounded anime.js, CSS, or WAAPI animation, uses Three.js without another finite duration source, or has no duration-bearing animation signal at all. npx hyperframes lint enforces this (root_composition_missing_duration_source). See the runtime/adapter-specific docs under hyperframes-animation/adapters/ for the full contract per runtime.

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 without explicit root data-duration.

Also avoid:

Animating anything outside the visual-property allowlist: opacity, transform components (translateX, translateY, scale, rotate, rotateX, rotateY, skew), color, backgroundColor, borderColor, borderRadius, CSS variables, media volume, and deterministic text counters. Never animate display or visibility; use opacity/transforms and timed clip visibility instead.
Multiple registered runtime instances owning the same CSS property on the same element at overlapping times.

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 / runtime instances 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: pretext.prepare(text, font) then pretext.layout(prepared, maxWidth, lineHeight). Pure arithmetic, ~0.0002 ms per call, safe for per-frame text reflow, shrinkwrap containers, and computing layout before render. fitTextFontSize is built on 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, resulting in invisible bars/fills. Give them display: block/inline-block/flex-item and a real width/height (e.g. width: 100% inside a sized parent). This is silent; lint/inspect 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. Otherwise they overlap a neighbor or get clipped. Position for the largest frame, not the resting one. This is silent.

Why This Matters

The renderer takes a time value and produces a pixel buffer. There is no notion of "playback"; every frame is a fresh seek. Any state that depends on having reached this frame through a prior frame (timers, accumulated state, event-driven animations) will desync when the renderer samples out of order or in parallel.

If you find yourself reaching for setTimeout, requestAnimationFrame, or addEventListener to drive a visual, rebuild it as a tween on the registered timeline instead.

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