> For the complete documentation index, see [llms.txt](https://nytshift.gitbook.io/nytshift-docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://nytshift.gitbook.io/nytshift-docs/design-decisions-and-proposals/css-architecture.md).

# CSS architecture and reviewed exceptions

The canonical source inventory is `apps/web/app/styles.css`. `scripts/split-route-css.mjs` deterministically emits independently loaded base, landing, command-deck, terminal and identity sheets. The source file is retained as the reviewable migration ledger; it is not imported by the application.

Initial decoded CSS budgets are 180,000 bytes per route. Terminal and identity interaction-only selectors are deferred until the first pointer or keyboard interaction. `pnpm css:check` rejects stale generated sheets, and the browser performance gate records actual route bytes separately.

## Breakpoint names

CSS custom properties cannot be used in media-query conditions, so reviewed numeric conditions are named here and enforced as an allowlist:

|   Width | Name            | Use                                     |
| ------: | --------------- | --------------------------------------- |
|   380px | micro           | exceptionally narrow phones             |
|   520px | compact-sheet   | dense sheets and emergency controls     |
|   600px | compact-card    | small command-deck cards                |
|   650px | mobile          | primary terminal and landing transition |
|   680px | customer-stack  | customer route cards                    |
|   720px | sheet           | wide mobile sheets                      |
|   760px | command-mobile  | command deck navigation and panels      |
|   820px | dossier-stack   | dossier and trust sections              |
|   900px | workspace-stack | chart and workspace toolbars            |
|   980px | tablet          | terminal rail/workspace transition      |
| 1,100px | command-desktop | command deck wide layout                |
| 1,180px | laptop          | landing and terminal laptop layout      |
| 1,360px | dense-desktop   | terminal density adjustment             |
| 1,440px | wide-desktop    | advanced-chart chrome                   |

New breakpoints require a named use case and visual evidence at the adjacent existing widths.

## `!important` exception policy

`!important` is permitted only for these reviewed categories:

* focus-visible and screen-reader-only accessibility utilities;
* reduced-motion guarantees that must override authored animation;
* operational typography and 44px mobile target floors that override legacy component shorthand;
* semantic positive, warning and negative status utilities;
* third-party/advanced-chart chrome visibility and containment overrides;
* legacy terminal error/readability corrections pending component-local CSS migration.

Generated-file occurrence ceilings are frozen in `route-css.test.ts`. They prevent silent growth; reducing a ceiling is always allowed. Adding an occurrence requires moving the rule into one of the categories above, documenting why normal cascade order is insufficient and reviewing affected desktop/mobile screenshots.


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