# Aspect ratio (`aspect-ratio`)

Media frame that locks width/height — images and embeds fill with object-fit cover.

> **Layer:** L1 surface · **Recipe:** _(unset — see docs/agent/pick-a-surface.md)_
> Curriculum: `AGENTS.md` · pick matrix: `docs/agent/pick-a-surface.md` · blast radius: `CONSUMER_MAP.md`

> **Dialect:** Partial below is **unprefixed** (gallery / standalone HM). DOM contract Python often uses the **source token** `data-dz-*` / `dz-*` (Dazzle dual-lock). Match the CSS/JS bundle you load.

> **Demo vs contract:** Live gallery behaviour may use `/mock/*` or flash toasts. Those are **offline demos only** — implement **Server exchange** + **DOM contract**, not the mock. See AGENTS.md › Gallery demos.

## Copy this

```html
<div class="hm-demo-row" style="gap: var(--space-md); align-items: flex-start;">
  <div class="aspect-ratio" data-ratio="1/1" style="width: 6rem;" aria-label="1:1 frame"><span style="display:grid;place-items:center;background:var(--colour-brand-soft);color:var(--colour-brand-text);font-size:var(--text-xs);">1:1</span></div>
  <div class="aspect-ratio" data-ratio="16/9" style="width: 10rem;" aria-label="16:9 frame"><span style="display:grid;place-items:center;background:var(--colour-brand-soft);color:var(--colour-brand-text);font-size:var(--text-xs);">16:9</span></div>
  <div class="aspect-ratio" data-ratio="4/3" style="width: 8rem;" aria-label="4:3 frame"><span style="display:grid;place-items:center;background:var(--colour-brand-soft);color:var(--colour-brand-text);font-size:var(--text-xs);">4:3</span></div>
</div>
```

## Server exchange

This Hyperpart has **no server exchange** — presentation or client chrome only. If you put `hx-*` on a control that uses this markup, that action's exchange belongs to the action, not this part.

## Swap contract

Agent-visible HTMX topology (ADR-0054 / decision 0012). **Exchange envelope** = what the response may re-emit relative to the persistent slot (`body_only` | `outer` | `none` | `host_owned` | `document`). Dual-lock validates part markup only — not this envelope. Stem: `stems/morph-safe-hypermedia.md`.

**No host HTMX exchange** on this part — presentation or client chrome only. **Exchange envelope:** `n/a`.

If a **host** wraps this markup in `hx-*`, **that host owns the swap contract** (sole identity + envelope). Prefer `innerMorph` / `outerMorph` for stable slots; replacement for flash; body-only responses under inner swaps.

### Envelope response examples

What the **server returns** for each exchange. Match the **exchange envelope**; dual-lock still applies to interior markup.

This part has no owned exchange (envelope `n/a`). If a host adds `hx-*`, that host’s envelope applies — typically `body_only`:

```html
<!-- Prefer hx-swap=innerMorph into a stable body slot -->
<div class="dz-stack">content…</div>
```

Do **not** re-own the slot:

```html
<div id="aspect-ratio-root" data-dz-region>…</div>
```

## How to use it

No extended guidance authored yet — start from Copy this and the dependency chips.

### Seams

- copy the partial under Copy this; keep root class and data-* modifiers so the CSS/JS bundle matches
- no Server exchange on this part — pure presentation or client chrome
- satisfy the DOM contract tables (CI stop-ship)

## DOM contract

What emitted markup must satisfy (CI: `tests/test_contracts.py`). Do not invent attrs outside the tables. Python modules under `contracts/` are **package-internal dual-locks** (`from contracts._kit import …`) — not FastAPI business handlers. App servers implement **Server exchange** endpoints; this section constrains the HTML those endpoints return.

### `contracts/aspect_ratio.py`

- **Required root:** `.dz-aspect-ratio` (part `aspect-ratio`)

| Node | Attr | Constraint |
|---|---|---|
| `.dz-aspect-ratio` | `—` | — |

#### Module source

Monorepo dual-lock only — import `contracts._kit` from the HM package. Do not paste into app route modules.

```python
"""HYPERPART: aspect-ratio — media frame that locks width/height.

Dual-lock unit is the frame root. ``data-dz-ratio`` presets and child
fill (object-fit) are host-owned. Class ``.dz-aspect-ratio`` is the stable
substrate root — FragmentRenderer ``AspectRatio`` + media-thumb field compose.
"""

from contracts._kit import DomContract, Node

DOM_CONTRACT = DomContract(
    part="aspect-ratio",
    root=".dz-aspect-ratio",
    nodes=(Node(".dz-aspect-ratio", attrs={}),),
)

__all__ = ["DOM_CONTRACT"]
```

## Notes

shadcn parity (HMC-036). Pure CSS aspect-ratio + data-dz-ratio presets (1/1, 4/3, 16/9, 21/9). Dual-lock root .dz-aspect-ratio (HMC-132). Dazzle: AspectRatio fragment + media field thumbs (logo_url/preview_url/photo_url). No controller.

## Source files

- `site/registry.py` (partial + exchanges + guidance)
- `contracts/aspect_ratio.py`