CSS Modern

Maintainable CSS with Grid, Flexbox, custom properties, container queries, cascade layers.

When to Use

Use when:

• Writing CSS styles for layouts and components
• Implementing responsive designs with Grid/Flexbox
• Using custom properties, container queries, @layer
• Creating animations and transitions

Don't use for Tailwind (tailwindcss skill) or MUI sx (mui skill).

Critical Patterns

✅ REQUIRED: Custom Properties for Theming

/* CORRECT */
:root {
  --color-primary: #0066cc;
  --spacing: 1rem;
}
.button {
  background: var(--color-primary);
  padding: var(--spacing);
}

/* WRONG: hardcoded values */
.button {
  background: #0066cc;
  padding: 1rem;
}

✅ REQUIRED: Grid/Flexbox Over Floats

/* CORRECT */
.container {
  display: flex;
  gap: 1rem;
  justify-content: space-between;
}

/* WRONG: floats */
.container {
  float: left;
  clear: both;
}

✅ REQUIRED: Respect prefers-reduced-motion

@media (prefers-reduced-motion: reduce) {
  * {
    animation-duration: 0.01ms !important;
    transition-duration: 0.01ms !important;
  }
}

✅ REQUIRED: Modern Responsive Patterns

/* Fluid typography */
h1 { font-size: clamp(2rem, 5vw, 4rem); }

/* Container queries */
@container (min-width: 400px) {
  .card { display: grid; grid-template-columns: 1fr 2fr; }
}

/* Modern aspect ratio (not padding hack) */
.video-container { aspect-ratio: 16 / 9; }

✅ REQUIRED: Modern Selectors

:is(h1, h2, h3) { color: var(--color-heading); }
:where(ul, ol) { padding-left: 1rem; }       /* zero specificity */
.card:has(img) { display: grid; }             /* parent selection */

✅ REQUIRED: Cascade Layers

@layer reset, base, components, utilities;

@layer reset {
  * { margin: 0; padding: 0; box-sizing: border-box; }
}
@layer base {
  body { font-family: system-ui, sans-serif; }
}
@layer components {
  .button { padding: 0.5rem 1rem; }
}
@layer utilities {
  .text-center { text-align: center; }
}

Decision Tree

One-dimensional layout?
  → Flexbox

Two-dimensional layout?
  → Grid

Responsive sizing?
  → clamp(), min(), max()

Theme values?
  → :root custom properties + var()

Center element?
  → Flexbox place-content: center or Grid place-items: center

Hide element?
  → display: none (removes), visibility: hidden (keeps space), opacity: 0 (transitions)

Responsive breakpoints?
  → Container queries (component), media queries (viewport)

Example

:root {
  --color-primary: #0066cc;
  --spacing-unit: 0.5rem;
}

.card {
  display: flex;
  flex-direction: column;
  gap: calc(var(--spacing-unit) * 2);
  padding: var(--spacing-unit);
  border-radius: 0.5rem;
  background-color: var(--color-primary);
}

@media (prefers-reduced-motion: reduce) {
  * { animation-duration: 0.01ms !important; }
}

Edge Cases

• Handle print stylesheets with @media print
• Support dark mode via prefers-color-scheme
• Test with different font sizes and zoom levels
• Verify with color blindness simulators
• Avoid !important except in utility layers

Checklist

• Custom properties for all theme values
• Grid/Flexbox for layout (no floats)
• prefers-reduced-motion respected
• @layer for cascade control
• Logical properties used (margin-inline, padding-block)
• Modern selectors (:is(), :where(), :has())
• clamp()/min()/max() for fluid sizing
• aspect-ratio for media containers
• BEM or consistent naming convention
• Tested across target browsers

Resources

• https://web.dev/learn/css/
• https://developer.mozilla.org/en-US/docs/Web/CSS