Web Interface Guidelines

---
name: web-interface-guidelines
description: Vercel's comprehensive UI guidelines for building accessible, performant web interfaces. Use this skill when reviewing or building UI components for compliance with best practices around accessibility, performance, animations, and visual stability.
---

# Web Interface Guidelines

Review UI code for compliance with Vercel's web interface standards.

## Quick Reference - Visual Stability

| Issue | Rule |
|-------|------|
| Images without dimensions | `<img>` needs explicit `width` and `height` (prevents CLS) |
| Font loading flash | Critical fonts: `<link rel="preload" as="font">` with `font-display: swap` |
| Large lists | Virtualize lists >50 items (`content-visibility: auto`) |
| Layout reads in render | No `getBoundingClientRect`, `offsetHeight` in render path |

## Full Rules

### Images

- `<img>` needs explicit `width` and `height` (prevents CLS)
- Below-fold images: `loading="lazy"`
- Above-fold critical images: `priority` or `fetchpriority="high"`

### Performance

- Large lists (>50 items): virtualize (`virtua`, `content-visibility: auto`)
- No layout reads in render (`getBoundingClientRect`, `offsetHeight`, `offsetWidth`, `scrollTop`)
- Batch DOM reads/writes; avoid interleaving
- Add `<link rel="preconnect">` for CDN/asset domains
- Critical fonts: `<link rel="preload" as="font">` with `font-display: swap`

### Accessibility

- Icon-only buttons need `aria-label`
- Form controls need `<label>` or `aria-label`
- Interactive elements need keyboard handlers (`onKeyDown`/`onKeyUp`)
- `<button>` for actions, `<a>`/`<Link>` for navigation (not `<div onClick>`)
- Images need `alt` (or `alt=""` if decorative)

### Focus States

- Interactive elements need visible focus: `focus-visible:ring-*` or equivalent
- Never `outline-none` / `outline: none` without focus replacement
- Use `:focus-visible` over `:focus` (avoid focus ring on click)

### Animation

- Honor `prefers-reduced-motion` (provide reduced variant or disable)
- Animate `transform`/`opacity` only (compositor-friendly)
- Never `transition: all`—list properties explicitly

### Forms

- Inputs need `autocomplete` and meaningful `name`
- Use correct `type` (`email`, `tel`, `url`, `number`) and `inputmode`
- Never block paste (`onPaste` + `preventDefault`)
- Labels clickable (`htmlFor` or wrapping control)

### Content Handling

- Text containers handle long content: `truncate`, `line-clamp-*`, or `break-words`
- Flex children need `min-w-0` to allow text truncation
- Handle empty states—don't render broken UI for empty strings/arrays

### Anti-patterns (flag these)

- `user-scalable=no` or `maximum-scale=1` disabling zoom
- `transition: all`
- `outline-none` without focus-visible replacement
- Images without dimensions
- Large arrays `.map()` without virtualization
- Form inputs without labels
- Icon buttons without `aria-label`