Web Platform Design Guidelines

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Web platform design and accessibility guidelines. Use when building web interfaces, auditing accessibility, implementing responsive layouts, or reviewing web UI code. Triggers on tasks involving HTML, CSS, web components, WCAG compliance, responsive design, or web performance.

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Framework-agnostic rules for accessible, performant, responsive web interfaces. Based on WCAG 2.2, MDN Web Docs, and modern web platform APIs.

1. Accessibility / WCAG [CRITICAL]

Accessibility is not optional. Most rules in this section map to WCAG 2.2 success criteria at Level A or AA. A small number of best-practice rules (noted inline) target Level AAA or go beyond WCAG.

1.1 Use Semantic HTML Elements

Use elements for their intended purpose. Semantic structure provides free accessibility, SEO, and reader-mode support.

Element	Purpose
`<main>`	Primary page content (one per page)
`<nav>`	Navigation blocks
`<header>`	Introductory content or navigational aids
`<footer>`	Footer for nearest sectioning content
`<article>`	Self-contained, independently distributable content
`<section>`	Thematic grouping with a heading
`<aside>`	Tangentially related content (sidebars, callouts)
`<figure>` / `<figcaption>`	Illustrations, diagrams, code listings
`<details>` / `<summary>`	Expandable/collapsible disclosure widget
`<dialog>`	Modal or non-modal dialog boxes
`<time>`	Machine-readable dates/times
`<mark>`	Highlighted/referenced text
`<address>`	Contact information for nearest article/body

<!-- Good -->
<main>
  <article>
    <h1>Article Title</h1>
    <p>Content...</p>
  </article>
  <aside>Related links</aside>
</main>

<!-- Bad: div soup -->
<div class="main">
  <div class="article">
    <div class="title">Article Title</div>
    <div class="content">Content...</div>
  </div>
</div>

Anti-pattern: Using <div> or <span> for interactive elements. Never write <div onclick> when <button> exists.

1.2 ARIA Labels on Interactive Elements

Every interactive element must have an accessible name. Prefer visible text; use aria-label or aria-labelledby only when visible text is insufficient (SC 4.1.2).

<!-- Icon-only button: needs aria-label -->
<button aria-label="Close dialog">
  <svg aria-hidden="true">...</svg>
</button>

<!-- Linked by labelledby -->
<h2 id="section-title">Notifications</h2>
<ul aria-labelledby="section-title">...</ul>

<!-- Redundant: visible text is enough -->
<button>Save Changes</button> <!-- No aria-label needed -->

1.3 Keyboard Navigation

All interactive elements must be reachable and operable via keyboard (SC 2.1.1).

Use native interactive elements (<button>, <a href>, <input>, <select>) which are keyboard-accessible by default.
Custom widgets need tabindex="0" to enter tab order and keydown handlers for activation.
Never use tabindex values greater than 0.
Trap focus inside modals; return focus on close.

// Focus trap for modal
dialog.addEventListener('keydown', (e) => {
  if (e.key === 'Tab') {
    const focusable = dialog.querySelectorAll(
      'button, [href], input, select, textarea, [tabindex]:not([tabindex="-1"])'
    );
    const first = focusable[0];
    const last = focusable[focusable.length - 1];
    if (e.shiftKey && document.activeElement === first) {
      e.preventDefault();
      last.focus();
    } else if (!e.shiftKey && document.activeElement === last) {
      e.preventDefault();
      first.focus();
    }
  }
});

1.4 Visible Focus Indicators

Never remove focus outlines without providing a visible replacement (SC 2.4.7, enhanced SC 2.4.11 (AA) and SC 2.4.12 (AAA) in WCAG 2.2).

/* Good: custom focus indicator */
:focus-visible {
  outline: 3px solid var(--focus-color, #4A90D9);
  outline-offset: 2px;
}

/* Remove default only when :focus-visible is supported */
:focus:not(:focus-visible) {
  outline: none;
}

/* Bad: removing all focus styles */
/* *:focus { outline: none; } */

WCAG 2.2 requires focus indicators to have a minimum area of the perimeter of the component times 2px, with 3:1 contrast against adjacent colors.

1.5 Skip Navigation Links

Provide a mechanism to skip repeated blocks of content (SC 2.4.1).

<body>
  <a href="#main-content" class="skip-link">Skip to main content</a>
  <nav>...</nav>
  <main id="main-content">...</main>
</body>

.skip-link {
  position: absolute;
  top: -100%;
  left: 0;
  z-index: 1000;
  padding: 0.75rem 1.5rem;
  background: var(--color-primary);
  color: var(--color-on-primary);
}
.skip-link:focus {
  top: 0;
}

1.6 Alt Text for Images

Every <img> must have an alt attribute (SC 1.1.1).

Informative images: describe the content and function. alt="Bar chart showing sales doubled in Q4".
Decorative images: use alt="" (empty string) so screen readers skip them.
Functional images (inside links/buttons): describe the action. alt="Search".
Complex images: use alt for short description, link to long description or use <figcaption>.

<img src="chart.png" alt="Revenue chart: Q1 $2M, Q2 $2.4M, Q3 $3.1M, Q4 $4.5M">
<img src="decorative-wave.svg" alt="">

1.7 Color Contrast

Maintain minimum contrast ratios (SC 1.4.3, 1.4.6, 1.4.11).

Content	Minimum Ratio
Normal text (<24px / <18.66px bold)	4.5:1
Large text (>=24px / >=18.66px bold)	3:1
UI components and graphical objects	3:1

Do not rely on color alone to convey information (SC 1.4.1). Pair color with icons, text, or patterns.

/* Check contrast of these tokens */
:root {
  --text-primary: #1a1a2e;    /* on white: ~16:1 */
  --text-secondary: #555770;  /* on white: ~6.5:1 */
  --text-disabled: #767693;   /* on white: ~4.5:1, borderline */
}

1.8 Form Labels

Every form input must have a programmatically associated label (SC 1.3.1, 3.3.2).

<!-- Explicit label (preferred) -->
<label for="email">Email address</label>
<input id="email" type="email" autocomplete="email">

<!-- Implicit label (acceptable) -->
<label>
  Email address
  <input type="email" autocomplete="email">
</label>

<!-- Never: placeholder as sole label -->
<!-- <input placeholder="Email"> -->

1.9 Error Identification

Identify and describe errors in text (SC 3.3.1). Link error messages to inputs with aria-describedby or aria-errormessage.

<label for="email">Email</label>
<input id="email" type="email" aria-describedby="email-error" aria-invalid="true">
<p id="email-error" role="alert">Enter a valid email address, e.g. name@example.com</p>

1.10 ARIA Live Regions

Announce dynamic content changes to screen readers (SC 4.1.3).

<!-- Polite: announced when user is idle -->
<div aria-live="polite" aria-atomic="true">
  3 results found
</div>

<!-- Assertive: interrupts current speech -->
<div role="alert">
  Your session will expire in 2 minutes.
</div>

<!-- Status messages -->
<div role="status">
  File uploaded successfully.
</div>

Use aria-live="polite" by default. Reserve role="alert" / aria-live="assertive" for time-sensitive warnings.

1.11 ARIA Role Quick Reference

Role	Purpose	Native Equivalent
`button`	Clickable action	`<button>`
`link`	Navigation	`<a href>`
`tab` / `tablist` / `tabpanel`	Tab interface	None
`dialog`	Modal	`<dialog>`
`alert`	Assertive live region	None
`status`	Polite live region	`<output>`
`navigation`	Nav landmark	`<nav>`
`main`	Main landmark	`<main>`
`complementary`	Aside landmark	`<aside>`
`search`	Search landmark	`<search>` (HTML5)
`img`	Image	`<img>`
`list` / `listitem`	List	`<ul>/<li>`
`heading`	Heading (with `aria-level`)	`<h1>`-`<h6>`
`menu` / `menuitem`	Menu widget	None
`tree` / `treeitem`	Tree view	None
`grid` / `row` / `gridcell`	Data grid	`<table>`
`progressbar`	Progress	`<progress>`
`slider`	Range input	`<input type="range">`
`switch`	Toggle	`<input type="checkbox">`

Rule: Prefer native HTML over ARIA. Use ARIA only when no native element exists for the pattern.

1.12 Label in Name (WCAG 2.5.3 Level A)

When an interactive element has visible text, its accessible name must contain that visible text as a substring (SC 2.5.3). Voice control users (Dragon NaturallySpeaking, macOS Voice Control) speak the visible label to activate controls. If aria-label replaces or contradicts the visible text, voice commands fail.

<!-- Correct: aria-label contains visible text as substring -->
<button aria-label="Delete item from cart">Delete</button>

<!-- Correct: no aria-label needed — visible text is the accessible name -->
<button>Save Changes</button>

<!-- Correct: icon button — no visible text, aria-label is fine -->
<button aria-label="Close dialog">
  <svg aria-hidden="true">...</svg>
</button>

<!-- Incorrect: aria-label overrides visible text with different text -->
<button aria-label="Remove">Delete</button>

<!-- Incorrect: aria-label does not contain visible "Submit" -->
<button aria-label="Proceed to next step">Submit</button>

Rule: When visible text is present, aria-label must include that visible text (verbatim, case-insensitively). Prefer no aria-label at all when visible text is sufficient.

2. Responsive Design [CRITICAL]

2.1 Mobile-First Approach

Write base styles for the smallest viewport. Layer complexity with min-width media queries.

/* Base: mobile */
.grid {
  display: grid;
  grid-template-columns: 1fr;
  gap: 1rem;
}

/* Tablet */
@media (min-width: 48rem) {
  .grid {
    grid-template-columns: repeat(2, 1fr);
  }
}

/* Desktop */
@media (min-width: 64rem) {
  .grid {
    grid-template-columns: repeat(3, 1fr);
  }
}

2.2 Fluid Layouts with Modern CSS Functions

Use clamp(), min(), and max() for fluid sizing without breakpoints.

/* Fluid typography */
h1 {
  font-size: clamp(1.75rem, 1.2rem + 2vw, 3rem);
}

/* Fluid spacing */
.section {
  padding: clamp(1.5rem, 4vw, 4rem);
}

/* Fluid container */
.container {
  width: min(90%, 72rem);
  margin-inline: auto;
}

2.3 Container Queries

Size components based on their container, not the viewport.

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

@container card (min-width: 400px) {
  .card {
    display: grid;
    grid-template-columns: 200px 1fr;
  }
}

@container card (min-width: 700px) {
  .card {
    grid-template-columns: 300px 1fr;
    gap: 2rem;
  }
}

2.4 Content-Based Breakpoints

Set breakpoints where your content breaks, not at device widths. Common starting points:

/* Content-based, not "iPhone" or "iPad" */
@media (min-width: 30rem)  { /* ~480px: single column gets cramped */ }
@media (min-width: 48rem)  { /* ~768px: room for 2 columns */ }
@media (min-width: 64rem)  { /* ~1024px: room for sidebar + content */ }
@media (min-width: 80rem)  { /* ~1280px: wide multi-column */ }

2.5 Touch Targets

Minimum 44x44 CSS pixels for touch targets (WCAG SC 2.5.5 AAA; SC 2.5.8 requires only 24x24px at AA). Provide at least 24px spacing between adjacent targets.

button, a, input, select, textarea {
  min-height: 44px;
  min-width: 44px;
}

/* Enlarge tap area without changing visual size */
.icon-button {
  position: relative;
  width: 24px;
  height: 24px;
}
.icon-button::after {
  content: "";
  position: absolute;
  inset: -10px; /* expands clickable area */
}

2.6 Viewport Meta Tag

Always include in the document <head>:

<meta name="viewport" content="width=device-width, initial-scale=1">

Never use maximum-scale=1 or user-scalable=no -- these break pinch-to-zoom accessibility (SC 1.4.4).

2.7 No Horizontal Scrolling

Content must reflow at 320px width without horizontal scrolling (SC 1.4.10).

/* Prevent overflow */
img, video, iframe, svg {
  max-width: 100%;
  height: auto;
}

/* Contain long words/URLs */
.prose {
  overflow-wrap: break-word;
}

/* Tables: scroll container, not page */
.table-wrapper {
  overflow-x: auto;
  -webkit-overflow-scrolling: touch;
}

3. Forms [HIGH]

3.1 Label Every Input

Every input needs a visible, programmatically associated label. See section 1.8.

3.2 Autocomplete Attributes

Use autocomplete for common fields to enable browser autofill (SC 1.3.5).

<input type="text" autocomplete="name" name="full-name">
<input type="email" autocomplete="email" name="email">
<input type="tel" autocomplete="tel" name="phone">
<input type="text" autocomplete="street-address" name="address">
<input type="text" autocomplete="postal-code" name="zip">
<input type="text" autocomplete="cc-name" name="card-name">
<input type="text" autocomplete="cc-number" name="card-number">
<input type="password" autocomplete="new-password" name="password">
<input type="password" autocomplete="current-password" name="current-pw">

3.3 Correct Input Types

Use the right type to trigger appropriate mobile keyboards and native validation.

Type	Use For
`email`	Email addresses
`tel`	Phone numbers
`url`	URLs
`number`	Numeric values with spinners (not for phone, zip, card numbers)
`search`	Search fields (shows clear button)
`date` / `time` / `datetime-local`	Temporal values
`password`	Passwords (triggers password manager)
`text` with `inputmode="numeric"`	Numeric data without spinners (PINs, zip codes)

<input type="tel" inputmode="numeric" pattern="[0-9]*" autocomplete="one-time-code">

3.4 Inline Validation

Validate on blur (not on every keystroke). Show success and error states.

<div class="field" data-state="error">
  <label for="username">Username</label>
  <input id="username" type="text" aria-describedby="username-hint username-error" aria-invalid="true">
  <p id="username-hint" class="hint">3-20 characters, letters and numbers only</p>
  <p id="username-error" class="error" role="alert">Username must be at least 3 characters</p>
</div>

.field[data-state="error"] input {
  border-color: var(--color-error);
  box-shadow: 0 0 0 1px var(--color-error);
}
.field[data-state="error"] .error { display: block; }
.field:not([data-state="error"]) .error { display: none; }

3.5 Fieldset and Legend for Groups

Group related inputs with <fieldset> and label the group with <legend>.

<fieldset>
  <legend>Shipping Address</legend>
  <label for="street">Street</label>
  <input id="street" type="text" autocomplete="street-address">
  <!-- ... -->
</fieldset>

<fieldset>
  <legend>Preferred contact method</legend>
  <label><input type="radio" name="contact" value="email"> Email</label>
  <label><input type="radio" name="contact" value="phone"> Phone</label>
</fieldset>

3.6 Required Field Indication

Indicate required fields visually and programmatically. Use required attribute and visible markers.

<label for="name">
  Full name <span aria-hidden="true">*</span>
  <span class="sr-only">(required)</span>
</label>
<input id="name" type="text" required autocomplete="name">

If most fields are required, indicate which are optional instead.

3.7 Submit Button State

Do not disable the submit button. Instead, validate on submit and show errors.

<!-- Good: always enabled, validate on submit -->
<button type="submit">Create Account</button>

<!-- Bad: disabled button with no explanation -->
<!-- <button type="submit" disabled>Create Account</button> -->

Disabled buttons fail to communicate why the user cannot proceed. If you must disable, provide a visible explanation.

3.8 Keep Instructions Near the Field

Place format examples, constraints, and recovery text next to the relevant field via hint and error text. Never explain requirements only once in introductory copy and expect users to remember them later.

4. Typography [HIGH]

4.1 Font Stacks

Use system font stacks for performance, or web fonts with proper fallbacks.

/* System font stack */
body {
  font-family: system-ui, -apple-system, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
}

/* Monospace stack */
code, pre, kbd {
  font-family: ui-monospace, "Cascadia Code", "Source Code Pro", Menlo, Consolas, monospace;
}

/* Web font with fallbacks and size-adjust */
@font-face {
  font-family: "CustomFont";
  src: url("/fonts/custom.woff2") format("woff2");
  font-display: swap;
  font-weight: 100 900;
}
body {
  font-family: "CustomFont", system-ui, sans-serif;
}

4.2 Relative Units

Use rem for font sizes and spacing. Use em for component-relative sizing.

html {
  font-size: 100%; /* = 16px default, respects user preference */
}

body {
  font-size: 1rem;       /* 16px */
}

h1 { font-size: 2.5rem; }  /* 40px */
h2 { font-size: 2rem; }    /* 32px */
h3 { font-size: 1.5rem; }  /* 24px */
small { font-size: 0.875rem; } /* 14px */

/* Never: font-size: 16px; (ignores user zoom settings) */

4.3 Line Height and Spacing

Body text line height of at least 1.5 (SC 1.4.12). Paragraph spacing at least 2x font size.

body {
  line-height: 1.6;
}

h1, h2, h3 {
  line-height: 1.2;
}

p + p {
  margin-top: 1em;
}

4.4 Maximum Line Length

Limit line length to approximately 75 characters for readability.

.prose {
  max-width: 75ch;
}

/* Or for a content column */
.content {
  max-width: 40rem; /* roughly 65-75ch depending on font */
  margin-inline: auto;
}

4.5 Typographic Details

Use real quotes, proper dashes, and tabular numbers for data.

/* Smart quotes */
q { quotes: "\201C" "\201D" "\2018" "\2019"; } /* curly double then single */

/* Tabular numbers for aligned data */
.data-table td {
  font-variant-numeric: tabular-nums;
}

/* Oldstyle numbers for running prose (optional) */
.prose {
  font-variant-numeric: oldstyle-nums;
}

/* Proper list markers */
ul { list-style-type: disc; }
ol { list-style-type: decimal; }

4.6 Heading Hierarchy

Use h1 through h6 in order. Never skip levels. One h1 per page.

<!-- Good -->
<h1>Page Title</h1>
  <h2>Section</h2>
    <h3>Subsection</h3>
  <h2>Another Section</h2>

<!-- Bad: skipping h2 -->
<h1>Page Title</h1>
  <h3>Subsection</h3> <!-- Where is h2? -->

If you need visual styling that differs from the hierarchy, use CSS classes:

<h2 class="text-lg">Visually smaller but semantically h2</h2>

5. Performance [HIGH]

5.1 Lazy Load Below-Fold Images

Use native lazy loading for images not visible on initial load.

<!-- Above fold: load eagerly, add fetchpriority -->
<img src="hero.webp" alt="Hero image" fetchpriority="high" width="1200" height="600">

<!-- Below fold: lazy load -->
<img src="feature.webp" alt="Feature image" loading="lazy" width="600" height="400">

5.2 Explicit Image Dimensions

Always specify width and height to prevent layout shift (CLS).

<img src="photo.webp" alt="Description" width="800" height="600">

/* Responsive images with aspect ratio preservation */
img {
  max-width: 100%;
  height: auto;
}

5.3 Resource Hints

Use preconnect for third-party origins and preload for critical resources.

<head>
  <!-- Preconnect to critical third-party origins -->
  <link rel="preconnect" href="https://fonts.googleapis.com">
  <link rel="preconnect" href="https://cdn.example.com" crossorigin>

  <!-- Preload critical resources -->
  <link rel="preload" href="/fonts/main.woff2" as="font" type="font/woff2" crossorigin>
  <link rel="preload" href="/css/critical.css" as="style">

  <!-- DNS prefetch for non-critical origins -->
  <link rel="dns-prefetch" href="https://analytics.example.com">
</head>

5.4 Code Splitting

Load JavaScript only when needed. Use dynamic import() for route-based and component-based splitting.

// Route-based splitting
const routes = {
  '/dashboard': () => import('./pages/dashboard.js'),
  '/settings':  () => import('./pages/settings.js'),
};

// Interaction-based splitting
button.addEventListener('click', async () => {
  const { openEditor } = await import('./editor.js');
  openEditor();
});

5.5 Virtualize Long Lists

For lists exceeding a few hundred items, render only visible rows.

// Concept: virtual scrolling
// Render only items in viewport + buffer
const visibleStart = Math.floor(scrollTop / itemHeight);
const visibleEnd = visibleStart + Math.ceil(containerHeight / itemHeight);
const buffer = 5;
const renderStart = Math.max(0, visibleStart - buffer);
const renderEnd = Math.min(totalItems, visibleEnd + buffer);

5.6 Avoid Layout Thrashing

Batch DOM reads and writes. Never interleave them.

// Bad: read-write-read-write (forces synchronous layout)
elements.forEach(el => {
  const height = el.offsetHeight;     // read
  el.style.height = height + 10 + 'px'; // write
});

// Good: batch reads, then batch writes
const heights = elements.map(el => el.offsetHeight); // all reads
elements.forEach((el, i) => {
  el.style.height = heights[i] + 10 + 'px'; // all writes
});

5.7 Use `will-change` Sparingly

Only apply will-change to elements that will animate, and remove it after animation completes.

/* Good: scoped and temporary */
.card:hover {
  will-change: transform;
}
.card.animating {
  will-change: transform, opacity;
}

/* Bad: blanket will-change */
/* * { will-change: transform; } */

5.8 Expose Waiting States Promptly

After a user action, acknowledge the new state immediately. If work cannot finish within a brief moment, show progress, skeletons, optimistic UI, or aria-busy feedback instead of leaving the interface unchanged.

6. Animation and Motion [MEDIUM]

6.1 Respect prefers-reduced-motion

Always provide a reduced-motion alternative (SC 2.3.3, Level AAA).

/* Define animations normally */
.fade-in {
  animation: fadeIn 300ms ease-out;
}

@keyframes fadeIn {
  from { opacity: 0; transform: translateY(8px); }
  to   { opacity: 1; transform: translateY(0); }
}

/* Remove or reduce for users who prefer it */
@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after {
    animation-duration: 0.01ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: 0.01ms !important;
    scroll-behavior: auto !important;
  }
}

// Check in JavaScript
const prefersReducedMotion = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

6.2 Compositor-Friendly Animations

Animate only transform and opacity for smooth 60fps animation. These run on the GPU compositor thread.

/* Good: compositor-only properties */
.slide-in {
  transition: transform 200ms ease-out, opacity 200ms ease-out;
}

/* Bad: triggers layout/paint */
.slide-in-bad {
  transition: left 200ms, width 200ms, height 200ms;
}

6.3 No Flashing Content

Never flash content more than 3 times per second (SC 2.3.1). This can trigger seizures.

6.4 Transitions for State Changes

Use transitions for hover, focus, open/close, and other state changes to provide continuity.

.dropdown {
  opacity: 0;
  transform: translateY(-4px);
  transition: opacity 150ms ease-out, transform 150ms ease-out;
  pointer-events: none;
}
.dropdown.open {
  opacity: 1;
  transform: translateY(0);
  pointer-events: auto;
}

6.5 Meaningful Motion Only

Animation should communicate state, guide attention, or show spatial relationships. Never animate for decoration alone.

7. Dark Mode and Theming [MEDIUM]

7.1 System Preference Detection

@media (prefers-color-scheme: dark) {
  :root {
    --bg: #0f0f17;
    --text: #e4e4ef;
    --surface: #1c1c2e;
    --border: #2e2e44;
  }
}

7.2 CSS Custom Properties for Theming

Define all theme values as custom properties. Toggle themes by changing property values.

:root {
  color-scheme: light dark;

Shortened here. Read the whole file on GitHub.

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Forks: 32
Last commit: Mar 2026

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Catalog kind: skill
Gateway key: web-design-guidelines-ehmo
Source: github.com/ehmo/platform-design-skills

github.com/ehmo/platform-design-skills

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