Responsive Layout

---
name: responsive-layout
description: Design and implement adaptive layouts using CSS Grid, Flexbox, container queries, and fluid typography/spacing — the craft layer for layouts that work correctly across all viewport sizes without JavaScript.
---

# Skill: responsive-layout

Load this skill when the primary task is designing or debugging a layout that
must work across breakpoints. Do not load it for routine margin or padding
adjustments on an already-responsive surface. Load `responsive-layout` when:

- Building the layout structure for a new page or section from scratch
- Debugging a layout that breaks at a specific viewport width
- Designing a shared component that must adapt to variable-width containers
- Evaluating whether a layout approach is correct before implementation

---

## Layout primitive selection

**CSS Grid:** for two-dimensional layouts and page-level structure. Use when
elements need to be positioned on both a row axis and a column axis, or when
the layout requires elements to align with other elements across rows/columns.

**Flexbox:** for one-dimensional alignment and component-level distribution.
Use when elements need to be distributed along a single axis (row or column),
with flexible sizing and alignment.

**The mixing rule:** never use Grid and Flexbox to solve the same axis
in the same container. Use Grid for the page layout; use Flexbox inside a
Grid cell for component-level distribution within that cell.

**Common mistakes:**
- Using Flexbox for a two-column layout with a sidebar — Grid is the right
  tool; Flexbox cannot express "sidebar is fixed-width, content fills the rest"
  without a `min-width: 0` hack on the content column.
- Using Grid for a single-axis distribution of buttons in a toolbar — Flexbox
  is the right tool; Grid's two-dimensional power is wasted here.

---

## Container queries vs. media queries

**Use container queries** when a component must adapt to the width of its
own container rather than the viewport. This is the correct tool for shared
components that appear in multiple layout contexts (e.g., a card that appears
in a sidebar at 320px and in a main content area at 600px).

```css
/* Define the containment context on the parent */
.card-container {
  container-type: inline-size;
  container-name: card;
}

/* Query the container, not the viewport */
@container card (min-width: 500px) {
  .card {
    display: grid;
    grid-template-columns: 120px 1fr;
  }
}
```

Container queries are Baseline Widely Available as of 2023.

**Use media queries** when layout changes are driven by the viewport — page-level
structure that changes how sections are arranged. Navigation, sidebar
visibility, and column count at the page level belong here.

```css
@media (min-width: 768px) {
  .page-layout {
    display: grid;
    grid-template-columns: 240px 1fr;
  }
}
```

**Decision rule:** if the layout change is for a shared component that appears
in multiple container widths — use container queries. If the layout change is
for the page structure that responds to the viewport — use media queries.

---

## Fluid typography

Use `clamp()` for type that scales smoothly between a minimum and maximum
viewport width without breakpoint jumps:

```css
/* clamp(minimum, preferred, maximum) */
/* Preferred: linear interpolation from min-size at min-viewport to max-size at max-viewport */

h1 {
  /* 24px at 320px viewport, scales to 48px at 1280px viewport */
  font-size: clamp(1.5rem, 2.5vw + 0.5rem, 3rem);
}

p {
  /* 16px at 320px viewport, scales to 18px at 1280px viewport */
  font-size: clamp(1rem, 0.5vw + 0.875rem, 1.125rem);
}
```

**Formula for the `vw + rem` preferred value:**
```
slope = (max-size - min-size) / (max-viewport - min-viewport)
y-intercept = min-size - (slope * min-viewport)
preferred = slope * 100vw + y-intercept
```

Derive min and max font sizes from the spacing scale (see `token-architecture`)
so typography and spacing scales remain proportional.

---

## Breakpoint strategy

Name breakpoints semantically — not by device. Device names encode specific
dimensions that change with hardware generations; semantic names are stable:

```css
/* Token-defined breakpoints — semantic names */
:root {
  --breakpoint-sm:  480px;
  --breakpoint-md:  768px;
  --breakpoint-lg:  1024px;
  --breakpoint-xl:  1280px;
  --breakpoint-2xl: 1536px;
}
```

**Mobile-first (min-width):** write base styles for mobile, then use
`@media (min-width: ...)` to enhance for larger viewports. Mobile-first
produces smaller files for mobile users (the base styles need no media query).

**Never encode specific device names** (`@media (max-width: 375px)` for
"iPhone SE") — those dimensions are historical snapshots that will be wrong
within 18 months.

---

## Grid patterns

**The 12-column grid** — flexible foundation for page layouts:
```css
.layout {
  display: grid;
  grid-template-columns: repeat(12, minmax(0, 1fr));
  gap: var(--ds-space-4);
}

/* A content area spanning 8 columns, centered */
.main-content {
  grid-column: 3 / span 8;
}
```

**`auto-fill` vs. `auto-fit`** for responsive card grids:
```css
/* auto-fill: preserves empty tracks (grid does not collapse) */
.card-grid-fill {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(240px, 1fr));
}

/* auto-fit: collapses empty tracks (cards expand to fill the row) */
.card-grid-fit {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(240px, 1fr));
}
```

Use `auto-fill` when preserving the column structure matters (e.g., aligning
with a fixed-column grid above); use `auto-fit` when cards should expand to
fill available space.

**Named grid areas** for semantic layouts:
```css
.page {
  display: grid;
  grid-template-areas:
    "header header"
    "sidebar main"
    "footer footer";
  grid-template-columns: 240px 1fr;
  grid-template-rows: auto 1fr auto;
}

.page-header  { grid-area: header; }
.page-sidebar { grid-area: sidebar; }
.page-main    { grid-area: main; }
.page-footer  { grid-area: footer; }
```

**`subgrid`** for aligned multi-column forms (Baseline Widely Available 2023):
```css
.form-row {
  display: grid;
  grid-template-columns: subgrid;
  grid-column: 1 / -1;
}
```

---

## The no-JS rule

Responsive layout must be fully functional without JavaScript. If a layout
requires JS to function at certain viewport sizes, it is a CSS architecture
problem, not a JS opportunity.

**CSS-only responsive patterns for common UI:**

**Hamburger nav (disclosure-pattern, CSS-only):**
Use `<details>` + `<summary>` for a native CSS-only toggle — no JS required.
```html
<details class="nav-disclosure">
  <summary>Menu</summary>
  <nav>…</nav>
</details>
```

**Card grid:** `auto-fill` / `auto-fit` with `minmax()` — no breakpoint JS.

**Table:** on narrow viewports, display each row as a block with
`display: block` on `<td>` elements and use `data-label` attributes
for column headers. This requires no JS.

---

## Output rendering

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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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## Common failures to refuse

| Pattern | Problem | Fix |
|---|---|---|
| Fixed pixel widths on containers (`width: 1200px`) | Container overflows viewport on narrower screens | Use `max-width` with `width: 100%` or a percentage |
| `overflow: hidden` on containers | Can hide content on small viewports that the user cannot scroll to | Audit whether `overflow: hidden` is actually needed; prefer `overflow: clip` for visual clipping without hiding content from assistive tech |
| Font sizes below 16px on mobile | Browsers may zoom automatically; can trigger CLS; fails readability standards | Minimum 16px (`1rem`) for body text on mobile |
| Content reordering that breaks tab order | Visual `order` property in Flexbox/Grid moves content visually but not in the DOM | DOM order must match visual order; use `order` only for cosmetic reordering within the same semantic group |
| Viewport units for font sizes without clamp | `font-size: 2vw` produces 0px at zero viewport width | Always wrap viewport-unit font sizes in `clamp()` |