web-ui-best-practices

# Web UI best practices

Principles for building web interfaces that feel fast, intentional, and respectful of the user's time. Every rule here is a smell test, violating one is fine if you have a reason, violating several means the UI needs work.

## Speed

Every interaction completes in under 100ms. If it can't, fake it.

- Optimistic UI updates, show the result before the server confirms
- Debounce inputs, but never debounce perceived response
- Prefetch likely next routes on hover or viewport entry
- Use `will-change` and `transform` for animations, never `top`/`left`
- Measure with `performance.now()`, not gut feel

```js
// Optimistic delete, remove from UI immediately, reconcile later
async function handleDelete(id) {
  setItems(prev => prev.filter(i => i.id !== id));
  try {
    await api.delete(`/items/${id}`);
  } catch {
    setItems(prev => [...prev, originalItem]);
    toast("Couldn't delete. Restored.");
  }
}
```

### Skeleton loading states

Never show a spinner when you know the shape of what's coming. Render a skeleton that matches the layout, then swap in real content.

```css
.skeleton {
  background: linear-gradient(90deg, #f0f0f0 25%, #e0e0e0 50%, #f0f0f0 75%);
  background-size: 200% 100%;
  animation: shimmer 1.5s infinite;
  border-radius: 4px;
}

@keyframes shimmer {
  0% { background-position: 200% 0; }
  100% { background-position: -200% 0; }
}
```

## Modern CSS toolkit

Four capabilities matured between 2023 and 2026 that change how you build component-level responsive layouts and SPA-like transitions without JavaScript. Reach for them before adding a framework.

### Container queries

Container queries let a component respond to **its container's** size, not the viewport's. The same card can render in a 300px sidebar and a 900px main column without media-query coordination at the page level.

```css
.card-list {
  container-type: inline-size;
  container-name: cards;
}

@container cards (min-width: 480px) {
  .card { display: grid; grid-template-columns: 120px 1fr; }
}
```

Stable in all major browsers since 2023. Replaces most "the same component in two places needs to look different" hacks.

### `:has()` parent selector

`:has()` lets a parent style itself based on its descendants, the long-requested "parent selector." Useful for marking a form field as in-error, a card as having an attached image, or a row as containing a focused input, all without JS.

```css
/* Highlight a form group when its input has focus */
.form-group:has(input:focus) {
  outline: 2px solid var(--color-primary);
}

/* Add bottom margin to articles that contain a figure */
article:has(figure) {
  margin-bottom: 2rem;
}
```

Stable in Chrome, Safari, and Firefox since late 2023. Cuts a real category of JS-driven class toggling.

### View transitions

The View Transitions API animates between two DOM states (route changes, modal open/close, list-item swaps) without a framework. The browser snapshots the old state, swaps in the new state, then crossfades or slides between them.

```js
// Same-document transition (Chrome 111+, Safari TP, Firefox behind a flag)
function navigate(newView) {
  if (!document.startViewTransition) {
    renderView(newView);
    return;
  }
  document.startViewTransition(() => renderView(newView));
}
```

```css
/* Smooth crossfade by default; override per element */
::view-transition-old(*) { animation-duration: 200ms; }
::view-transition-new(*) { animation-duration: 200ms; }
```

Cross-document view transitions (between full page navigations) shipped to Chrome 126 in 2024 and let MPAs feel like SPAs. Pair with `prefers-reduced-motion` so users with motion sensitivity get an instant swap, not an animation.

### Scroll-driven animations

`animation-timeline: scroll()` and `animation-timeline: view()` drive CSS animations from scroll position instead of wall-clock time. The classic use case is a progress indicator at the top of an article that fills as you scroll.

```css
@keyframes fill { from { transform: scaleX(0); } to { transform: scaleX(1); } }

.read-progress {
  position: fixed; top: 0; left: 0; right: 0; height: 3px;
  background: var(--color-primary);
  transform-origin: left;
  animation: fill linear;
  animation-timeline: scroll(root);
}
```

Stable in Chromium-based browsers (Chrome 115+, Edge); not yet in Safari or Firefox as of 2026-05. Use as progressive enhancement; provide a JS fallback or accept a less-flashy baseline elsewhere.

## No product tours

If you need a tour to explain your UI, the UI is wrong. Instead:

- Empty states that teach by doing ("Create your first project")
- Progressive disclosure, show features when they become relevant
- Inline hints that disappear after first use
- Defaults that work without configuration

## URLs

Slugs are short, readable, and human-guessable. No UUIDs, no query param soup.

```
Good:  /projects/weather-app
       /settings/billing
       /docs/api/auth

Bad:   /projects/550e8400-e29b-41d4-a716-446655440000
       /app?view=settings&tab=billing&subsection=plan
       /dashboard#!/module/documents/list?filter=active
```

- Use slugs derived from user-provided names
- Keep nesting to 3 segments max
- Make URLs copyable and shareable, they are the product's memory

## Persistent resumable state

Users leave and come back. Respect that.

- Save draft form state to `localStorage` or the server
- Restore scroll position on back navigation
- Preserve filter/sort selections across sessions
- URL encodes the current view state, sharing a URL reproduces the view

```js
// Persist form state across sessions
function usePersistentForm(key, defaults) {
  const [state, setState] = useState(() => {
    const saved = localStorage.getItem(key);
    return saved ? JSON.parse(saved) : defaults;
  });

  useEffect(() => {
    localStorage.setItem(key, JSON.stringify(state));
  }, [key, state]);

  return [state, setState];
}
```

## Color restraint

Not more than 3 colors. One primary, one accent, one for danger/des