Compose Video

HTML is the source of truth for video. A composition is an HTML file with data-* attributes for timing, a GSAP timeline for animation, and CSS for appearance. The framework handles clip visibility, media playback, and timeline sync.

Approach

Before writing HTML, think at a high level:

1. What — what should the viewer experience? Identify the narrative arc, key moments, and emotional beats.
2. Structure — how many compositions, which are sub-compositions vs inline, what tracks carry what (video, audio, overlays, captions).
3. Timing — which clips drive the duration, where do transitions land, what's the pacing.
4. Execute — then implement using the rules below.

For small edits (fix a color, adjust timing, add one element), skip straight to the rules.

When no visual-style.md or animation direction is provided, follow house-style.md for motion defaults, sizing, and color palettes.

Data Attributes

All Clips

data-track-index does not affect visual layering — use CSS z-index.

Composition Clips

Composition Structure

Every composition is a <template> wrapping a <div> with data-composition-id. Each must include its own GSAP script and register its timeline:

<template id="my-comp-template">
  <div data-composition-id="my-comp" data-width="1920" data-height="1080">
    <!-- content -->
    <style>
      [data-composition-id="my-comp"] {
        /* scoped styles */
      }
    </style>
    <script src="https://cdn.jsdelivr.net/npm/gsap@3.14.2/dist/gsap.min.js"></script>
    <script>
      window.__timelines = window.__timelines || {};
      const tl = gsap.timeline({ paused: true });
      // tweens...
      window.__timelines["my-comp"] = tl;
    </script>
  </div>
</template>

Load in root: <div id="el-1" data-composition-id="my-comp" data-composition-src="compositions/my-comp.html" data-start="0" data-duration="10" data-track-index="1"></div>

Video and Audio

Video must be muted playsinline. Audio is always a separate <audio> element:

<video
  id="el-v"
  data-start="0"
  data-duration="30"
  data-track-index="0"
  src="video.mp4"
  muted
  playsinline
></video>
<audio
  id="el-a"
  data-start="0"
  data-duration="30"
  data-track-index="2"
  src="video.mp4"
  data-volume="1"
></audio>

Timeline Contract

• All timelines start { paused: true } — the player controls playback
• Register every timeline: window.__timelines["<composition-id>"] = tl
• Framework auto-nests sub-timelines — do NOT manually add them
• Duration comes from data-duration, not from GSAP timeline length
• Never create empty tweens to set duration

Rules (Non-Negotiable)

Deterministic: No Math.random(), Date.now(), or time-based logic. The renderer must produce identical output every time.

GSAP: Only animate visual properties (opacity, x, y, scale, rotation, color, backgroundColor, borderRadius, transforms). Do NOT animate visibility, display, or call video.play()/audio.play().

Animation conflicts: Never animate the same property on the same element from multiple timelines simultaneously — causes flickering in headless renders.

Never do:

1. Forget window.__timelines registration
2. Use video for audio — always muted video + separate <audio>
3. Nest video inside a timed div — use a non-timed wrapper
4. Use data-layer (use data-track-index) or data-end (use data-duration)
5. Animate video element dimensions — animate a wrapper div
6. Call play/pause/seek on media — framework owns playback
7. Create a top-level container without data-composition-id

Editing Existing Compositions

• Read the full composition first — match existing fonts, colors, animation patterns
• Only change what was requested — don't rewrite untouched sections
• Don't rewrite entire files for small changes
• Preserve timing of unrelated clips

Typography and Assets

• Every composition loads its own fonts (@import or @font-face in <style>)
• Use font-display: block for local fonts — renderer needs fonts loaded before capturing
• Add crossorigin="anonymous" to media loaded from external URLs
• Minimum readable text: 20px landscape, 18px portrait
• For dynamic text that may overflow its container, use window.__hyperframes.fitTextFontSize(text, { maxWidth, fontFamily, fontWeight }) to compute the largest font size that fits on one line. Returns { fontSize, fits }. Options: baseFontSize (default 78), minFontSize (default 42), step (default 2). fontFamily and fontWeight must match the CSS applied to the element.
• All files (video, audio, fonts, images) live at the project root alongside index.html
• From sub-compositions, use ../ to reference root files

For PiP, title cards, and slide show patterns, see patterns.md. For data, stats, and infographics, see data-in-motion.md. For typewriter text and other GSAP animation effects, see the gsap-effects skill. For audio-driven animation (beat sync, glow, pulse), see the audio-reactive skill.

Output Checklist

• Every top-level container has data-composition-id
• Every composition has data-width, data-height, data-duration
• Compositions in own HTML files, loaded via data-composition-src
• <template> wrapper on sub-compositions
• window.__timelines registered for every composition
• 100% deterministic — no randomness
• Each composition includes GSAP: <script src="https://cdn.jsdelivr.net/npm/gsap@3.14.2/dist/gsap.min.js"></script>
• npx hyperframes lint passes with 0 errors
• npx hyperframes validate passes with 0 errors (run both before opening the studio)