# `<LED>`

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## Purpose

`<LED>` is the output indicator control. It accepts a numeric value in
the standard `BoundControlProps` domain and renders it as a coloured dot
whose brightness is driven either by a boolean threshold (`mode="boolean"`)
or a continuous mapping (`mode="continuous"`). The component is read-only:
it does not invoke `onChange` because indicators are downstream consumers
of bindings, not upstream sources.

The visual rendering pulls from the skin's CSS custom properties
(`--cs-led-on-color`, `--cs-led-off-color`) and uses `color-mix()` to
interpolate between them in continuous mode. This keeps brightness purely
a CSS computation — no per-frame inline-style churn — so the LED can be
driven from an animation frame loop without re-rendering React.

## Features

- Boolean and continuous brightness modes.
- Skin-driven colour via CSS custom properties.
- Accessible `role="status"` with `aria-live="polite"`.
- Default aria-label includes on/off state and percentage.
- Pure `ledBrightness` helper for unit testing the value clamp.

## Usage: Basic

```tsx
<LED value={1} min={0} max={1} onChange={() => undefined} label="Power" />
```

## Usage: Advanced

```tsx
<LED
    value={signalLevel}
    min={0}
    max={1}
    onChange={() => undefined}
    mode="continuous"
    size={24}
    ariaLabel="Signal-present indicator"
    className="signal-led"
/>
```

## API Reference

- `<LED>` props: `BoundControlProps & { mode?: "boolean" | "continuous", size?, className?, style? }`.
- `ledBrightness(value, min, max, mode)` — pure helper returning the
  normalised brightness in `[0, 1]`.

## Implementation Notes

`color-mix(in srgb, ...)` is widely supported in modern browsers but
absent from Safari Technology Preview prior to v16; consumers targeting
legacy engines should override the inline `background` via a skin or
custom `style`. The component intentionally does NOT animate brightness
transitions — animation is left to the consumer's stylesheet because
audio-meter LEDs typically need ballistic curves driven outside React.