# CSS Module Boundaries

## Objective

Keep each component's styles its own, and make the shared parts deliberate.

A boundary around each component's styles, exposing only a named set of hooks that callers are permitted to style through.

## 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. Identify the primitives the rest of the app builds on — button, field, card, menu, table — and treat everything inside them as private unless it is deliberately published.
2. For each primitive, publish an explicit surface: the variants it accepts, the slots a caller may target, and the custom properties it reads. Anything not on that list is internal and may change without notice.
3. Find the places where product screens currently reach into a primitive's internals and rewrite them to use the published surface, adding a variant or a slot where the need is legitimate.
4. Make the boundary enforceable rather than advisory, so a reach-in selector is caught during review or in the build instead of being discovered when a primitive is refactored months later.
5. Do not attempt to solve precedence here. Which rule wins when two collide belongs to CSS Layer Strategy; this feature decides who is allowed to write the second rule at all.

## 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

- Selectors that reach past a component's root into its internal elements must be rejected, because they silently couple a screen to markup the primitive's author is free to change.
- Theming needs a defined way in. Publish the named custom properties a component reads and let callers set those, rather than leaving people to guess and target internal class names.
- Composition between modules must not become circular. A chain in which one component's styles compose from another that composes back produces an order that depends on import sequence and will differ between builds.
- Product screens overriding a primitive's internals is the exact failure this feature exists to stop, and it must be treated as a defect rather than a shortcut, even when the override is small and works today.
- Generated or utility class names that appear inside a primitive are still internal; a caller matching on them has reached in by another route.
- Some overrides are genuinely necessary before the right variant exists. Give them a marked, temporary escape hatch that is visible in a search, rather than pretending the need does not arise.
- A component rendering into a portal or an overlay leaves its parent's scope, so styles that relied on ancestry will stop applying and must be passed in deliberately.

## 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 shared primitive has a documented public surface of variants, slots, and custom properties.
- [ ] No product screen selector targets a primitive's internal elements.
- [ ] Theming happens through published custom properties rather than internal class names.
- [ ] Module composition has no cycles and produces the same result regardless of import order.
- [ ] Boundary violations are caught by review or by the build, not at runtime.
- [ ] Temporary overrides are marked and findable by search.
- [ ] 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.