Modern CSS

Modern CSS

Modern CSS rules for creating robust, responsive and accessible UIs.

The rules work best when you apply a progressive enhancement approach. The CSS features are within Baseline Newly Available. Thanks to Interop, most are within Widely Available.

When editing existing files, match the surrounding code's style. For new code, follow the rules below. Where two rules could both apply, apply both consistently rather than picking one.

Rules

Architecture

Organizing styles (@layer)

• Rule: Use @layer to organize groups of styles.
• Constraint: Avoid global styles outside of layers.
• Rationale: Prevents style conflicts through managing specificity.
• References: @layer on MDN.
• Example:

/* avoid */
a {
  text-decoration-skip-ink: auto;
}

/* prefer */
@layer elements, components;

@layer elements {
  a {
    text-decoration-skip-ink: auto;
  }
}

Encapsulating styles (@scope)

• Rule: Use @scope to encapsulate styles.
• Constraint: Avoid custom element and components styles that are outside of a scope.
• Rationale: Prevents styles from bleeding into other components because selectors are scoped to the component.
• References: @scope on MDN.
• Example:

@scope (.card) {
  h2 {
    font-size: var(--large);
  }
}

Limiting scope (@scope ... to)

• Rule: Use a scope limit (@scope (outer) to (inner)) when a scoped component hosts content it doesn't own — embedded components, slot content, or user content.
• Constraint: Avoid base @scope over a component that holds slot content, embedded components, or user-generated HTML.
• Rationale: Stops the outer scope's styles bleeding into nested components or projected content; descendants past the limit keep their own scope's styles. Sometimes called donut scoping.
• References: @scope on MDN.
• Example:

/* avoid */
@scope (.card) {
  a {
    color: var(--blue);
  }
}

/* prefer */
@scope (.card) to (.content) {
  a {
    color: var(--blue);
  }
}

Nesting rules and at-rules (&)

• Rule: Use & for nesting rules and at-rules.
• Constraint: Avoid unnested selectors (e.g. a {} a:hover {}).
• Rationale: Clarifies the relationship between the nested selector and the parent selector.
• References: & nesting selector on MDN.
• Example:

a {
  color: var(--blue);

  &:hover {
    text-decoration: underline;
  }

  @container (width > 20em ) {
    place-self: center;
  }
}

Relational styles (:has())

• Rule: Use :has() for relational styles.
• Constraint: Avoid .has--like class names (e.g. .has-img {}).
• Rationale: The relationship lives in CSS, with no JS or build-time class toggling needed when the DOM changes.
• References: :has() on MDN.
• Example:

.card {
  &:has(img) {
    grid-template-rows: auto 1fr;
  }
}

Additive properties (:not() and 20em < width <= 40em)

• Rule: Use :not() and ranged queries (e.g. @media (20em < width <= 40em)) to create additive styles.
• Constraint: Avoid overriding styles (e.g. div { margin: 1rem; &:first-child { margin-block-start: 0; } })
• Rationale: Simplifies the mental model because you don't have to keep track of which styles are being overridden.
• References: :not() on MDN, media query range syntax on MDN.
• Example:

.card {
  /* Apply to all conditions */
  color: red;

  /* Apply based on the selector */
  &:not(:first-child) {
    margin-block-start: var(--medium);
  }

  /* Apply based on non-overlapping container conditions */
  @container example (width <= 20em) {
    background-color: var(--primary);
  }

  @container example (20em < width <= 40em ) {
    background-color: var(--secondary);
  }

  @container example (width > 40em ) {
    background-color: var(--tertiary);
  }
}

Typography

Fluid type sizes (clamp())

• Rule: Use clamp() for font sizes to create harmonious rhythmic scales that are appropriate to the screen size, e.g. Major Second (1.125) on narrow viewports and Major Third (1.25) wide ones.
• Constraint: Avoid fixed font sizes (e.g., px, rem) and central values without a rem addition (e.g. clamp(1.75rem, 5cqi, 2.25rem)).
• Rationale: Ensures text is appropriately sized across different viewport sizes and can be zoomed for accessibility.
• References: clamp() on MDN, Responsive design: seams & edges. and Designing with fluid type scales .
• Example:

:root {
  --medium: clamp(1.3125rem, 1.1821rem + 0.6522cqi, 1.6875rem);
  --large: clamp(1.75rem, 1.5761rem + 0.8696cqi, 2.25rem);
}

Widow and orphan words (text-wrap)

• Rule: Use text-wrap with pretty or balance to avoid widow and orphan words.
• Constraint: Avoid default wrapping outside of inputs and text areas.
• Rationale: Improves the readability and aesthetics of text blocks.
• References: text-wrap on MDN.
• Example:

h1,
h2,
h3 {
  text-wrap: balance;
}

p {
  text-wrap: pretty;
}

Colors

Perceptual uniform lightness (oklch())

• Rule: Use oklch() for all colors.
• Constraint: Avoid hex, rgb(), hsl() and other color formats.
• Rationale: Easier to maintain perceptual uniform lightness to ensure text is accessible regardless of background color.
• References: oklch() on MDN.
• Example:

/* avoid */
:root {
  --success: #2d7a3e;
  --danger: hsl(0deg 70% 40%);
}

/* prefer */
:root {
  --success: oklch(40% 0.15 150deg);
  --danger: oklch(40% 0.2 25deg);
}

Respecting color preferences (color-scheme)

• Rule: Use color-scheme and light-dark() to support color schemes.
• Constraint: Avoid hardcoding colors that don't adapt to light and dark modes.
• Rationale: Improves accessibility by respecting a person's preference for light or dark mode.
• References: color-scheme on MDN, light-dark() on MDN.
• Example:

body {
  color-scheme: light dark;
  background-color: light-dark(oklch(98% 0.03 250deg), oklch(14% 0 0deg));
}

Relative color functions (oklch(from /* .. */) & color-mix() )

• Rule: Use relative color syntax (e.g. oklch(from var(--primary) l + 10%)) and functions (e.g. color-mix()) to create color relationships.
• Constraint: Avoid hardcoding colors that relate to other colors.
• Rationale: Creates a cohesive color palette that is easier to maintain and adjust.
• References: relative colors on MDN, color-mix() on MDN.
• Example:

button {
  background-color: var(--primary);

  &:hover {
    background-color: oklch(from var(--primary) l c calc(h - 10deg));
  }
}

Layout

Flow-relative layout (*-inline-*, *-block-*, cqi/vi, start/end)

• Rule: Use flow-relative properties (e.g. padding-block-start, inset-inline, inline-size), units (e.g. cqi, cqb, vi), and keywords (e.g. text-align: start) for layout.
• Constraint: Avoid physical equivalents (e.g. padding-top, width, cqw, vw, text-align: left).
• Rationale: Flexbox and grid already use inline/block axes; flow-relative layout keeps the rest of the box model consistent with them, and adapts automatically when writing mode or text direction changes.
• References: Logical properties and values on MDN.
• Example:

/* avoid */
.card {
  padding-top: 1rem;
  font-size: 20vw;
  text-align: left;
}

/* prefer */
.card {
  padding-block-start: 1rem;
  font-size: 20vi;
  text-align: start;
}

Container queries and units (@container, cqi etc.)

• Rule: Use container queries and units (e.g. cqi, cqb) for responsive layouts.
• Constraint: Avoid fixed units for spacing (e.g. padding-block: 16px, margin-inline: 1rem) as they create hard edges and seams.
• Rationale: Improves modularity and reusability as components adapt to their container size, and work across all device sizes not just a few.
• References: container on MDN, container query length units on MDN, Responsive design: seams & edges. and Designing with fluid type scales .
• Example:

.card {
  container: card / inline-size;
  padding: 2cqi;

  p {
    @container card (width > 30cqi) {
      place-self: center;
    }
  }
}

Intrinsic sizing (*-content)

• Rule: Use intrinsic sizing (e.g. max-inline-size: fit-content, block-size: max-content).
• Constraint: Avoid fixed sizes (e.g. width: 300px, height: 200px) for content elements.
• Rationale: Improves flexibility and prevents overflow issues as content determines its own size.
• References: fit-content on MDN, max-content on MDN.
• Example:

nav {
  max-inline-size: fit-content;
}

Aligning nested grids (subgrid)

• Rule: Use subgrid on grid-template-rows or grid-template-columns to align nested grid items with an ancestor grid.
• Constraint: Avoid fixed heights, JS measurement, or flattening the DOM to make sibling grids align.
• Rationale: Sibling components keep their internal markup while still aligning at parent grid boundaries; without subgrid, alignment falls back to fixed heights, JS measurement, or flattened DOM.
• References: subgrid on MDN.
• Example:

.card {
  display: grid;
  grid-template-rows: auto auto auto 1fr;

  > .content {
    display: grid;
    grid-row: span 4;
    grid-template-rows: subgrid;
  }
}

Motion

Respecting motion preferences (prefers-reduced-motion)

• Rule: Use prefers-reduced-motion: no-preference when applying large animations and transitions.
• Constraint: Avoid prefers-reduced-motion: reduce.
• Rationale: Treats motion as opt-in: the absence of animation is the default, so no fallback is needed for users who haven't expressed a preference. Inverting this (@media (prefers-reduced-motion: reduce)) requires every animation to also ship a reduce-motion override, and it's easy to miss one.
• References: prefers-reduced-motion on MDN.
• Example:

/* avoid */
.hero {
  animation: bounce-in 0.5s ease;
}

@media (prefers-reduced-motion: reduce) {
  .hero {
    animation: none;
  }
}

/* prefer */
@media (prefers-reduced-motion: no-preference) {
  .hero {
    animation: bounce-in 0.5s ease;
  }
}

Applying these rules

When asked to author, refactor, or review CSS:

1. Identify which rules apply to the task.
2. Read surrounding files to see which rules are already in use; match their patterns.
3. Apply the rules consistently across the change set, not just at the touch points.

Gotchas

Non-obvious traps that the rules above don't surface on their own:

• clamp() central values need a rem term: clamp(1.75rem, 1.5761rem + 0.8696cqi, 2.25rem), not clamp(1.75rem, 5cqi, 2.25rem).
• Ranged queries must not overlap: width <= 20em / 20em < width <= 40em / width > 40em, not width < 20em / width >= 20em.
• prefers-reduced-motion: no-preference opts motion in, reduce opts motion out. Reach for no-preference.
• oklch() not hsl() for theme colors. HSL's lightness channel isn't perceptually uniform.
• :has() makes .has-img-style classes obsolete. Don't add a class to track a relationship the DOM already expresses.
• grid-template-rows: subgrid only inherits tracks the child claims. Pair it with grid-row: span N so the child occupies the parent rows; without span, the subgrid child gets one row and aligns with nothing.

Examples

This site follows the rules:

• src/styles.css — @layer
• src/variables.css — oklch(), light-dark(), clamp()
• src/elements.css — &, text-wrap, prefers-reduced-motion, cqi
• src/components/signpost/signpost.css — @scope

When the skill is active, prefer matching patterns from the user's own files over fetching these examples.