Css Architecture

---
name: css-architecture
description: Organize CSS at scale using cascade layers, scoping strategies, and specificity budgets — preventing specificity wars, enabling safe deletion, and making CSS that other engineers can reason about.
---

# Skill: css-architecture

Load this skill when setting up CSS architecture for a new codebase, or
auditing and refactoring CSS in an existing one where specificity conflicts,
hard-to-predict cascade behavior, or unsafe deletion are active problems.
Do not load it for routine component styling on a codebase that already has
a working architecture. Load `css-architecture` when:

- Starting a new project and deciding the CSS organization strategy
- A codebase has accumulated specificity conflicts that produce unpredictable
  cascades
- Engineers cannot safely delete a CSS rule because they don't know what
  will break
- The codebase has grown beyond one developer and CSS changes produce
  unexpected regressions

---

## Cascade layers

`@layer` (Cascade Layers) is Baseline Widely Available as of 2022. It makes
the cascade explicit and controllable: a rule in a higher layer always wins
over a rule in a lower layer, regardless of specificity.

**Canonical layer order:**

```css
/* Declare all layers upfront — this is the only place layer order is defined */
@layer reset, base, theme, components, utilities, overrides;
```

| Layer | Contents | Notes |
|---|---|---|
| `reset` | Browser default reset (e.g., box-sizing, margins) | No project-specific styles |
| `base` | HTML element defaults (body font, heading scale, link style) | Unclassed element selectors only |
| `theme` | Design token custom properties (`:root {}` token block) | No component styles |
| `components` | Component class selectors (`.btn`, `.card`, `.modal`) | The main application CSS lives here |
| `utilities` | Single-purpose utility classes (`hidden`, `sr-only`, spacing helpers) | High-frequency reuse; should have low specificity |
| `overrides` | Escape hatch for third-party library overrides, print styles | Rarely used; document every rule added here |

**Why ordering is the invariant:** a rule in `utilities` always beats a rule
in `components`, regardless of how specific the component rule is. This means
utilities are reliable (you can always use a utility to override a component
style), but components cannot override utilities. The layer order must be
declared once at the top of the main stylesheet — never redeclare it in
component files.

**Importing into layers:**
```css
@layer reset {
  @import url('/css/reset.css');
}

@layer components {
  @import url('/css/button.css');
  @import url('/css/card.css');
}
```

---

## Scoping strategy

Choose one scoping strategy for the project. Mixing strategies in the same
codebase produces conflicting specificity assumptions.

**CSS Modules:** class names are hashed at build time (`button_abc123`);
guaranteed uniqueness per module. Appropriate for component-based frameworks
(React, Vue, Svelte) where each component file has its own CSS module.

- Strengths: zero naming collisions, dead code is detectable by the bundler.
- Weakness: no global shared classes unless explicitly exposed; requires a
  bundler.

**BEM (Block-Element-Modifier):** naming convention that encodes component
structure in the class name: `.block__element--modifier`.

- Strengths: portable, no build tools required, self-documenting.
- Weakness: class names become verbose; convention is manually enforced.

**Utility-first CSS:** all styling done with single-purpose utility classes.
No component-level CSS; styles are composed in HTML.

- Strengths: no dead CSS, highly predictable, fast iteration.
- Weakness: HTML becomes verbose; component abstraction is in JS, not CSS.

**CUBE CSS (Composition, Utility, Block, Exception):** a hybrid that combines
global composition (layout), utilities, block-level component styles, and
per-instance exception overrides.

**Selection criteria:**

| Criterion | CSS Modules | BEM | Utility-first | CUBE |
|---|---|---|---|---|
| Framework-based (React/Vue) | Best | Works | Works | Works |
| No build tools | No | Best | Requires CDN build | Works |
| Small team, fast velocity | Works | Works | Best | Works |
| Large team, strict naming | Works | Best | Works | Works |
| Design-system-driven | Works | Works | Works | Best |

---

## Specificity budget

**The rule of thumb:** no selector should exceed specificity `0-2-0`
(two class names). Selectors with higher specificity are hard to override
without writing even-higher-specificity selectors, producing a cascade war.

```css
/* Right: specificity 0-1-0 */
.button { color: var(--btn-text); }

/* Right: specificity 0-2-0 */
.button.is-active { color: var(--btn-text-active); }

/* Wrong: specificity 0-3-0 — already too high */
.nav .button.is-active { }

/* Wrong: ID selector — specificity 1-0-0 — overrides all class selectors */
#submit-button { }
```

**ID selectors in CSS are banned.** IDs have specificity `1-0-0` — they beat
every class-based selector regardless of how specific. Use IDs for JavaScript
hooks and accessibility (`aria-labelledby`, `<label for="...">`), never for
styling.

**To audit current specificity:** paste a stylesheet into a specificity
visualizer (specifishity.com or equivalent). Any selector above `0-2-0` is a
candidate for refactoring.

---

## Safe deletion check

Before deleting a CSS rule, answer three questions:

1. **Is this selector referenced in HTML?** Search the codebase for the class
   name (exact match, not substring). If no HTML uses the class, it is safe
   to delete.
   ```bash
   grep -r "class-name-here" --include="*.html" --include="*.jsx" --include="*.tsx" .
   ```

2. **Is this rule overriding something that would now cascade through?** Check
   whether the rule's declarations suppress a style from a lower layer. If
   deleted, the lower-layer style takes effect. This is not always wrong, but
   must be verified.

3. **Does the selector appear in tests or a style guide?** Visual regression
   tests and component showcase snapshots may reference class names indirectly.
   Search for the class name in test files.

If all three checks are clean, the rule is safe to delete. If any check is
ambiguous, leave the rule and add a comment explaining what it does.

---

## Token compliance audit

The same grep from `frontend-engineering` GATES step 3:

```bash
grep -E "#[0-9a-fA-F]{3,6}|rgba?\(|hsl\(|[0-9]+px" <file.css>
```

Output should return **only** the `:root` / primitive token-definition block.
Any hardcoded colour or spacing value outside that block is a violation.

**What counts as a violation:**
- Any `color`, `background-color`, `border-color`, or `fill` value expressed
  as a hex, `rgb()`, `rgba()`, or `hsl()` function outside the `:root` block.
- Any `padding`, `margin`, `gap`, `width`, or `height` value expressed as a
  pixel value that is not a token (e.g., `margin: 13px`).

**What does NOT count as a violation:**
- Primitive token definitions inside `:root {}`.
- `transparent`, `currentColor`, `inherit` — these are relational, not literal.
- `1px` for borders and outlines — a single-pixel border is a design primitive,
  not a spacing token.

---

## CSS custom property gotchas

**`@property` for type-safe tokens:**
```css
@property --ds-color-primary {
  syntax: '<color>';
  inherits: true;
  initial-value: #5e6ad2;
}
```
`@property` gives the browser the type of a custom property, enabling
interpolation in transitions and preventing values from being set to the
wrong type. Baseline Newly Available — check support requirements.

**Inheritance behavior:** CSS custom properties inherit by default through
the DOM. A token set on `:root` is available everywhere. If a component
needs to override a token for its own tree without affecting siblings, scope
the override to the component's root element:
```css
.my-component {
  --ds-color-primary: var(--ds-color-secondary);
}
```

**The isolation case (`initial`):** when a component must be fully isolated
from its parent's token environment (e.g., an iframe-embedded widget, a
third-party component), use `@layer` isolation and set custom properties to
their `initial` values explicitly.

**`!important` on a custom property:** CSS `!important` applies to the
property declaration, not the custom property value. It works but produces
the same cascade problems as `!important` on any other property — use it
only as a documented last resort in the `overrides` layer.

---

## Naming system

The functional naming rule applies to the entire CSS class name system as well
as token names.

**Role names survive redesigns; implementation names do not:**

| Wrong (implementation) | Right (role/function) |
|---|---|
| `.blue-button` | `.btn-primary` |
| `.large-font-header` | `.page-heading` |
| `.left-sidebar` | `.sidebar` (or `.layout-sidebar`) |
| `.two-column-layout` | `.content-layout` |

A `.blue-button` class becomes incorrect when the brand color changes to
green. A `.btn-primary` class is correct regardless of the brand color —
it describes the button's role in the hierarchy, not its current color.

Apply the same rule to CSS custom properties — see `token-architecture` for
the full naming system.

## Output rendering

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Lead with the useful outcome or next action. Use warm, non-blaming language and everyday words. Define an unfamiliar term in a few plain words before naming it; keep proper names and exact technical terms intact.
During tool work, do not narrate routine calls. Send an update only for safety, a blocker, a needed decision, a material scope change, a long wait, or an active host requirement.
When requesting input, ask only for what is needed now. Ask dependent questions one at a time; otherwise group related questions. Offer no more than three clear choices when choices help.
Shape the answer to the facts: one fact needs one sentence; related facts use prose; separate items use bullets; real sequences use numbered steps.
For prose artifacts, use descriptive headings, short resumable sections, one fact per sentence, and no repeated summary. Emphasize at most one load-bearing point per section. Group long inventories instead of truncating them.
Make the result stand alone. Do needed arithmetic, give real dates or times, and say what a file or link establishes instead of making the reader inspect it.
For code and comments, prefer obvious structure and names. Comment on intent, constraints, or trade-offs that the code cannot state clearly.
Use a table, tree, flow, or other visual only when it makes a relationship materially easier to understand.
Report the current state, not the path taken. Omit dead ends, resolved trade-offs, hedges, and advice the user did not request.
When editing maintained prose, consolidate repeated rules and navigation before adding another caveat.
Silence and brevity never reduce the work, checks, or requested coverage. Preserve depth, evidence, constraints, warnings, code, diffs, errors, and exact names, paths, and counts.
Keep verification compact: pass or fail, count, and runtime. Name a suite when it failed or when the name changes what the reader should do.
Before sending, check that the reader can act without counting, converting, opening a file, or asking what a line means.
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