---
title: Data Attributes
description: "Core attributes for controlling element timing and behavior."
---

Hyperframes uses HTML data attributes to control timing, media playback, and [composition](/concepts/compositions) structure. These are the declarative building blocks of every video.

## Timing Attributes

| Attribute | Example | Description |
|-----------|---------|-------------|
| `data-start` | `"0"` or `"intro"` | Start time in seconds, or a clip ID reference for [relative timing](#relative-timing) |
| `data-duration` | `"5"` | Duration in seconds. Required for images. Optional for video/audio (defaults to source duration). On the **root** composition it sets the total render length (see [Composition Attributes](#composition-attributes)); on nested sub-compositions it is ignored (their length comes from their child timeline). |
| `data-track-index` | `"0"` | Timeline track number. Controls z-ordering (higher = in front) and groups clips into rows. Clips on the same track cannot overlap. |

## Media Attributes

| Attribute | Example | Description |
|-----------|---------|-------------|
| `data-media-start` | `"2"` | Media playback offset / trim point in seconds. Default: `0` |
| `data-volume` | `"0.8"` | Audio/video volume, 0 to 1 |
| `data-has-audio` | `"true"` | Indicates video has an audio track |

## Composition Attributes

| Attribute | Example | Description |
|-----------|---------|-------------|
| `data-composition-id` | `"root"` | Unique ID for [composition](/concepts/compositions) wrapper (required on every composition) |
| `data-width` | `"1920"` | Composition width in pixels |
| `data-height` | `"1080"` | Composition height in pixels |
| `data-duration` (root) | `"30"` | On the **root** composition, the total render length / frame count in seconds. Read once from the source HTML at compile time, like `data-width` / `data-height`, so a script or `hyperframes render --variables` cannot change it (author it directly, one value per output). If the root omits `data-duration`, and only then, the renderer derives the total length from the live DOM / timeline after scripts run. |
| `data-composition-src` | `"./intro.html"` | Path to external [composition](/concepts/compositions) HTML file |
| `data-variable-values` | `'{"title":"Hello"}'` | JSON object of values passed to a nested composition. Inside the sub-composition, read them via `window.__hyperframes.getVariables()` — the runtime layers these over the sub-comp's own `data-composition-variables` defaults and exposes the merged result on a per-instance basis (the same source can be embedded multiple times with different values). |
| `data-composition-variables` | `'[{"id":"title","type":"string","label":"Title","default":"Hello"}]'` | JSON array of declared variables (`id`, `type`, `label`, `default`). Drives Studio editing UI and provides defaults read by `window.__hyperframes.getVariables()`. The CLI flag `hyperframes render --variables '<json>'` overrides these defaults at top-level render time; host elements override them per-instance via `data-variable-values`. |

## Element Visibility

Add `class="clip"` to all timed elements so the runtime can manage their visibility lifecycle:

```html index.html
<h1 id="title" class="clip"
    data-start="0" data-duration="5" data-track-index="0">
  Hello World
</h1>
```

## Relative Timing

Instead of calculating absolute start times, a clip can reference another clip's `id` in its `data-start` attribute. This means "start when that clip ends":

```html index.html
<video id="intro" data-start="0" data-duration="10" data-track-index="0" src="..."></video>
<video id="main" data-start="intro" data-duration="20" data-track-index="0" src="..."></video>
<video id="outro" data-start="main" data-duration="5" data-track-index="0" src="..."></video>
```

`main` resolves to second 10, `outro` resolves to second 30. If `intro`'s duration changes, downstream clips shift automatically.

### Offsets (Gaps and Overlaps)

Add `+ N` or `- N` after the ID to offset from the end of the referenced clip:

```html index.html
<!-- 2-second gap after intro -->
<video id="scene-a" data-start="intro + 2" data-duration="20"
       data-track-index="0" src="..."></video>

<!-- 0.5-second overlap with intro (crossfade) -->
<video id="scene-b" data-start="intro - 0.5" data-duration="20"
       data-track-index="1" src="..."></video>
```

<Note>
  Overlapping clips must be on different tracks -- clips on the same track cannot overlap in time.
</Note>

<Accordion title="Relative timing rules and constraints">
  **Same composition only** -- references resolve within the clip's parent [composition](/concepts/compositions). You cannot reference a clip in a sibling or parent composition.

  **No circular references** -- A cannot start after B if B starts after A. The resolver detects cycles and throws an error.

  **Referenced clip must have a known duration** -- either an explicit `data-duration` or a duration inferred from source media. If the referenced clip has no known duration, the reference cannot resolve.

  **Parsing rules** -- if the value is a valid number, it is treated as absolute seconds. Otherwise it is parsed as one of:
  - `<id>` -- start when that clip ends
  - `<id> + <number>` -- start N seconds after that clip ends
  - `<id> - <number>` -- start N seconds before that clip ends

  **Chain length** -- references can chain (`A` -> `B` -> `C`), but deeply nested chains make the timeline harder to reason about. Keep chains under 3-4 levels for readability.
</Accordion>

## Next Steps

<CardGroup cols={2}>
  <Card title="Compositions" icon="layer-group" href="/concepts/compositions">
    How compositions use data attributes to define video structure
  </Card>
  <Card title="HTML Schema Reference" icon="book" href="/reference/html-schema">
    Complete attribute reference with per-element details
  </Card>
  <Card title="GSAP Animation" icon="wand-magic-sparkles" href="/guides/gsap-animation">
    Animate elements alongside data-attribute-driven timing
  </Card>
  <Card title="Common Mistakes" icon="triangle-exclamation" href="/guides/common-mistakes">
    Pitfalls to avoid when setting up timing and attributes
  </Card>
</CardGroup>
