Agent Skills: More CSS

More CSS

Make intentional decisions about structure, tokens, and tooling so CSS scales across components, themes, teams, and time.

No frameworks. Do not use TailwindCSS, UnoCSS, Bootstrap, or any other CSS framework. Write vanilla CSS only. If a utility layer is needed, build it — don't import it.

Core Principles

1. Tokens first — Define values once as custom properties; reference everywhere
2. Layer everything — Use @layer to make specificity predictable and override-safe
3. Name with intent — Naming conventions exist to communicate, not to decorate
4. Tooling serves you — Use a bundler to split files and handle imports; never let tooling substitute for clear CSS architecture
5. Composition over inheritance — Build components from tokens and utilities, not from each other
6. Relative units — Use relative values for everything except border-width and outline-width; px anywhere else overrides user preferences

Architecture

Layer Order

Always declare layers upfront to make load-order explicit:

@layer config, resets, components, utilities, overrides;

• config — All design tokens as custom properties; no selectors
• resets — Normalize browser inconsistencies and element-level defaults (typography, links, form elements)
• components — Scoped, self-contained UI pieces
• utilities — Single-purpose helpers (u-visuallyhidden, u-truncate)
• overrides — Context-specific tweaks, third-party overrides

File Structure

Organize files to mirror the layer order:

styles/
├── config/
│   ├── color.css        # Color custom properties
│   ├── spacing.css      # Spacing scale
│   ├── typography.css   # Font sizes, weights, families
│   └── index.css        # Imports all config files
├── resets.css
├── components/
│   ├── button.css
│   └── card.css
├── utilities.css
└── main.css             # @import or @layer everything

Design Tokens

Use custom properties as your single source of truth. Define them in the config layer so every other layer can reference them.

Use OKLCH for Colors

Define all color tokens in OKLCH. It has a wider gamut than sRGB, perceptually uniform lightness (so oklch(40% ...) is always visibly darker than oklch(60% ...)), and makes generating hover/muted variants predictable — just adjust the L value.

/* Don't do this — hex values aren't perceptually predictable */
--color-bg-primary: #0066cc;
--color-bg-primary-hover: #0052a3; /* how much darker is this? */

/* Do this — lightness is explicit and adjustable */
--color-bg-primary: oklch(50% 0.2 260);
--color-bg-primary-hover: oklch(43% 0.2 260); /* clearly 7% darker */

This also makes dark mode token overrides straightforward — flip lightness, keep chroma and hue.

Naming Convention

Use a --[category]-[variant]-[modifier] pattern. For color tokens, use text, bg, or border as the variant — no other color groupings:

@layer config {
  :root {
    color-scheme: light dark;

    /* Color — text */
    --color-text-default: light-dark(oklch(15% 0 0), oklch(92% 0 0));
    --color-text-muted: light-dark(oklch(45% 0 0), oklch(65% 0 0));
    --color-text-on-primary: oklch(99% 0 0);
    --color-text-primary: oklch(50% 0.2 260);

    /* Color — bg */
    --color-bg-default: light-dark(oklch(98% 0 0), oklch(15% 0 0));
    --color-bg-subtle: light-dark(oklch(95% 0 0), oklch(20% 0 0));
    --color-bg-primary: oklch(50% 0.2 260);
    --color-bg-primary-hover: oklch(43% 0.2 260);

    /* Color — border */
    --color-border-default: light-dark(oklch(80% 0 0), oklch(35% 0 0));
    --color-border-primary: oklch(50% 0.2 260);

    /* Spacing */
    --space-1: 0.25rem;
    --space-2: 0.5rem;
    --space-3: 0.75rem;
    --space-4: 1rem;
    --space-6: 1.5rem;
    --space-8: 2rem;
    --space-12: 3rem;
    --space-16: 4rem;

    /* Typography */
    --font-family: system-ui, sans-serif;
    --font-family-code: ui-monospace, monospace;
    --font-size-sm: 0.875rem;
    --font-size-md: 1rem;
    --font-size-lg: 1.125rem;
    --font-size-xl: 1.25rem;
    --font-size-2xl: 1.5rem;
    --font-weight-normal: 400;
    --font-weight-medium: 500;
    --font-weight-bold: 700;
    --line-height-tight: 1.25;
    --line-height-base: 1.5;

    /* Border */
    --border-radius-sm: 0.25rem;
    --border-radius-md: 0.5rem;
    --border-radius-lg: 0.75rem;
    --border-width: 1px;

    /* Shadow */
    --shadow-sm: 0 0.0625rem 0.125rem oklch(0% 0 0 / 0.08);
    --shadow-md: 0 0.25rem 0.5rem oklch(0% 0 0 / 0.1);
    --shadow-lg: 0 0.5rem 1.5rem oklch(0% 0 0 / 0.12);

    /* Motion */
    --duration-fast: 150ms;
    --duration-base: 250ms;
    --duration-slow: 400ms;
    --easing-default: ease;
    --easing-spring: cubic-bezier(0.34, 1.56, 0.64, 1);
  }
}

Theming with Tokens

Define each theme-aware token once using light-dark() and let the browser switch automatically based on color-scheme. For user-togglable themes, just flip color-scheme — no token values need to be repeated:

@layer config {
  /* User-toggled theme — light-dark() reads color-scheme automatically */
  [data-theme="light"] {
    color-scheme: light;
  }
  [data-theme="dark"] {
    color-scheme: dark;
  }
}

Units

Font Sizes in rem, Not px

rem scales with the user's browser default font size and system text-size preferences. px ignores both — a user who sets their browser to 20px large text gets no benefit from your font-size: 16px declaration.

/* Don't do this — px overrides the user's preferences */
--font-size-md: 16px;
.c-heading { font-size: 24px; }

/* Do this — rem respects the browser default */
--font-size-md: 1rem;
--font-size-2xl: 1.5rem;

All font-size tokens must be in rem. Never override them with px in components.

Fluid Type with clamp() and Container Query Units

For headings and display text that should scale with available space, use clamp() with a container query unit (cqi) as the ideal value. cqi is 1% of the container's inline size — it responds to the element's actual container, not the viewport, so type in a narrow card column scales correctly even when the viewport is wide.

rem must remain the anchor for the minimum value so the type hierarchy still works when a user zooms to 200%+.

/* Fluid heading: scales between 1.25rem and 3rem based on container width */
h1 {
  font-size: clamp(1.25rem, 5cqi, 3rem);
}

cqi requires the parent to establish a containment context:

.c-card {
  container-type: inline-size;
}

Use vb as the fluid value only when no containment context exists and the type genuinely scales with the viewport block axis. For most UI components, cqi is the right choice.

Fluid Values for Layouts

Fluid layouts adapt to available space without hard-coded breakpoints. Prefer intrinsic sizing over media queries — let the content and container determine when things wrap or resize.

Preferred units for layout:

• % and fr — proportional sizing within grid and flex containers
• vi / vb / dvi / dvb — logical viewport units (inline and block axis); prefer these over vw/vh so layouts work across writing modes and directions
• ch — character-width unit; ideal for setting minimum column widths based on readable content width
• min(), max(), clamp() — responsive sizing in a single declaration without a breakpoint
• rem — spacing tokens are already in rem; use them for gaps and padding

border-width and outline-width are the only px exceptions — hairline borders and focus rings should stay crisp at a fixed thickness regardless of zoom. Everything else (shadows, spacing, radii) uses relative units.

Grid: auto-fit + minmax()

repeat(auto-fit, minmax()) creates a self-wrapping column grid with no media queries. Columns expand to fill space (auto-fit) and wrap when they'd drop below the minimum (minmax()).

.c-grid {
  display: grid;
  gap: var(--space-4);
  grid-template-columns: repeat(auto-fit, minmax(20ch, 1fr));
}

Use ch for the minimum — it ties the breakpoint to readable content width rather than an arbitrary pixel value. Use a component-scoped custom property to make it overridable per instance:

.c-grid {
  --c-grid-col-min: 20ch;
  grid-template-columns: repeat(auto-fit, minmax(var(--c-grid-col-min), 1fr));
}

Use auto-fill instead of auto-fit when you want empty columns to preserve their space (e.g. keeping a grid locked to N columns even when fewer items are present).

Flexbox: Deconstructed Pancake

For flex layouts where items should wrap independently at their own threshold, use flex: 1 1 <min>. Each item grows and shrinks freely but won't collapse below the minimum before wrapping.

.c-list {
  display: flex;
  flex-wrap: wrap;
  gap: var(--space-4);
}

.c-list > * {
  flex: 1 1 20ch;
}

This allows ch units (unlike the flex albatross calc() trick) and lets each item wrap independently rather than all items breaking at once.

Naming Conventions

Component Classes

Use BEM (Block, Element, Modifier) for components — it's verbose but unambiguous at scale:

/* Block */
.c-card {
}

/* Element (part of the block) */
.c-card__header {
}
.c-card__body {
}
.c-card__footer {
}

/* Modifier (variant of block or element) */
.c-card--featured {
}
.c-card__header--compact {
}

Class Prefixes

Use the c- prefix for components, u- for utilities, and js- for JavaScript hooks (never styled).

State classes use is- / has- (e.g. .is-loading, .has-error), but prefer ARIA attribute selectors where possible (e.g. [aria-expanded="true"], [aria-disabled="true"]).

<div class="c-card c-card--featured js-expandable">
  <div class="c-card__header">...</div>
</div>

Component CSS

Structure

Each component file should follow a consistent internal structure:

@layer components {
  /* 1. Block */
  .c-button {
    /* Layout */
    display: inline-flex;
    align-items: center;
    gap: var(--space-2);
    padding-block: var(--space-2);
    padding-inline: var(--space-4);

    /* Typography */
    font-family: var(--font-family);
    font-size: var(--font-size-md);
    font-weight: var(--font-weight-medium);
    line-height: var(--line-height-tight);

    /* Visual */
    background-color: var(--color-bg-primary);
    color: var(--color-text-on-primary);
    border: var(--border-width) solid transparent;
    border-radius: var(--border-radius-md);

    /* Interaction */
    cursor: pointer;
    transition: background-color var(--duration-fast) var(--easing-default);
  }

  /* 2. States */
  .c-button:hover {
    background-color: var(--color-bg-primary-hover);
  }

  .c-button:focus-visible {
    outline: 2px solid var(--color-border-primary);
    outline-offset: 0.125rem;
  }

  [aria-disabled="true"].c-button,
  .c-button:disabled {
    opacity: 0.5;
    cursor: not-allowed;
  }

  /* 3. Modifiers */
  .c-button--ghost {
    background-color: transparent;
    border-color: var(--color-border-primary);
    color: var(--color-text-primary);
  }

  .c-button--sm {
    padding-block: var(--space-1);
    padding-inline: var(--space-3);
    font-size: var(--font-size-sm);
  }
}

Only token references inside components

Components reference tokens — they never hardcode values. If you find yourself writing a raw color or a raw value inside a component, define a token for it first. This applies to focus styles too — always reference a token for the outline color so it stays on-brand and themeable.

Use :focus-visible (not :focus) for keyboard-only focus rings.

Quick Reference

| Situation | Pattern | | ----------------------- | -------------------------------------------------- | | Defining a color value | Token in @layer config | | Building a UI component | .c- class in @layer components | | One-off helper | .u- class in @layer utilities | | JS needs a hook | .js- class, never styled | | Component state | ARIA attribute selector, then .is-/.has- | | Theming / dark mode | light-dark() in tokens; color-scheme to toggle | | Font sizes | rem tokens only — never px | | Layout sizing | %, fr, vi/vb/dvi/dvb, min()/clamp(), rem — px only for border-width and outline-width |

References

Read these when you need more detail than the guidelines above:

• architecture.md - Read when designing the full CSS architecture for a large project, including token systems, file structure, and layer strategies
• modern-css.md - Read when you need specifics on modern CSS features, browser support for user preference queries, advanced typography options, or CSS tooling recommendations