# Adaptive Media Source Component

## Objective

Serve each image and video at a size and format the viewer's device can actually use.

One media component that chooses source, format, and dimensions from the rendered box and the browser's declared support.

## Before You Begin

This feature is being added to an application that already exists and already
works. Do not scaffold a new project, and do not assume a blank slate.

Inspect the codebase first and establish:

- The existing application structure and where code of this kind already lives.
- The framework and version in use.
- The existing design system — colours, spacing, typography, and component conventions.
- Existing UI components you can reuse instead of writing new ones.
- The existing database structure, if this feature needs to persist anything.
- The existing authentication and authorization system, if this feature is user-scoped.
- Dependencies already installed, so you don't add a library that duplicates one.
- The existing test setup and conventions.

Only start writing code once you understand the above. If the application
already implements part of this feature, extend it rather than replacing it.

## Implementation Instructions

1. Find every place the app renders a raw image or video element and route them all through a single component, so source selection is decided in one place rather than per template.
2. Derive the candidate widths from the sizes the element is genuinely rendered at across the app's breakpoints, and describe those sizes to the browser accurately — an inaccurate size hint is worse than none.
3. Always render an intrinsic width and height or an aspect ratio, so the box occupies its final space before any bytes arrive.
4. Offer modern formats with a defined fallback order, and confirm the fallback renders when the preferred format is unsupported or its derivative is missing.
5. Do not generate the derivatives here. Producing and storing sized, compressed files belongs to the Visual Asset Optimization Pipeline brief; this component only chooses among derivatives that already exist and falls back to the original when one does not.

## UI and UX Requirements

Match the application's existing design system exactly. Reuse its components,
spacing, and typography. This feature should look like it was always there.

## Responsive Requirements

Works on mobile, tablet, and desktop. Touch targets are large enough to hit on a
phone, and nothing overflows horizontally at 320px.

## Accessibility Requirements

- Fully keyboard navigable.
- Correct semantic elements and ARIA roles.
- Visible focus states.
- Meets WCAG AA contrast.
- Dynamic changes are announced to screen readers.
- Respects prefers-reduced-motion.

## Edge Cases

- Space must be reserved before the media loads. Without intrinsic dimensions, every image on the page pushes the content below it down as it arrives, and a reader loses their place mid-sentence.
- Private and access-controlled media must stay private. A transformation or resizing URL must not become an unauthenticated public read of an asset the viewer would otherwise need permission to see.
- A device with a high pixel ratio will request enormous sources unless the ladder is capped. Clamp the effective multiplier so a phone does not download a four-thousand-pixel image to fill a thumbnail.
- A transformed source that returns an error or an unsupported format must fall back to the original rather than leaving a broken element in the layout.
- Video needs a poster image sized by the same rules, and autoplay must be suppressed where the viewer has asked to reduce motion or is on a metered connection.
- Decorative media takes an empty alternative text so it is skipped by screen readers; meaningful media must carry text that survives the source swapping.
- Where a crop differs by breakpoint rather than just resolution, the component must select a different crop, not merely a smaller version of the wide one.

## Testing

Exercise the feature end to end in the running application. Cover every edge case
above, then run the existing test suite and confirm nothing regressed.

## Acceptance Criteria

- [ ] Every image and video in the app renders through the one component.
- [ ] Each element reserves its final dimensions before the source loads.
- [ ] The width served matches the rendered size, with a capped multiplier for high-density screens.
- [ ] An unsupported format or a missing derivative falls back to the original with nothing visibly broken.
- [ ] Access-controlled media cannot be read through a transformation URL without authorization.
- [ ] Alternative text is preserved regardless of which source is chosen.
- [ ] Derivative generation is delegated to the asset pipeline rather than reimplemented in the component.
- [ ] The feature matches the existing design system.
- [ ] No existing functionality is broken.

## Adaptation Rules

- Match the existing design system. Do not introduce a new colour palette,
  spacing scale, or component library.
- Reuse existing components and utilities wherever they fit.
- Follow the naming, file layout, and code style already present.
- Do not upgrade, replace, or remove existing dependencies to make this
  feature fit. Adapt the feature to the app, not the app to the feature.
- Do not break existing functionality. If a change is genuinely required in
  existing code, make the smallest one that works and say so.
- If something in these instructions conflicts with how the application is
  built, follow the application and explain the deviation.

## Final Verification

Before you report the work as done:

1. Re-read the acceptance criteria above and check each one against what you
   actually built.
2. Run the application and exercise the feature end to end.
3. Run the existing test suite and confirm you have broken nothing.
4. Check the feature on mobile, tablet, and desktop widths.
5. Check keyboard navigation and focus handling.
6. Summarize what changed: files added, files modified, and anything you
   deliberately did differently because of how this application is built.

If any acceptance criterion is unmet, fix it before reporting completion.