UI

Details that make interfaces feel better

Great interfaces rarely come from a single thing. It's usually a collection of small details that compound into a great experience. Apply these principles when building or reviewing UI code.

Quick Reference

Core Principles

1. Concentric Border Radius

Outer radius = inner radius + padding. Mismatched radii on nested elements is the most common thing that makes interfaces feel off.

2. Optical Over Geometric Alignment

When geometric centering looks off, align optically. Buttons with icons, play triangles, and asymmetric icons all need manual adjustment.

3. Shadows Over Borders

Layer multiple transparent box-shadow values for natural depth. Shadows adapt to any background; solid borders don't.

4. Interruptible Animations

Use CSS transitions for interactive state changes — they can be interrupted mid-animation. Reserve keyframes for staged sequences that run once.

5. Split and Stagger Enter Animations

Don't animate a single container. Break content into semantic chunks and stagger each with ~100ms delay.

6. Subtle Exit Animations

Use a small fixed translateY instead of full height. Exits should be softer than enters.

7. Contextual Icon Animations

Animate icons with opacity, scale, and blur instead of toggling visibility. Use exactly these values: scale from 0.25 to 1, opacity from 0 to 1, blur from 4px to 0px. If the project has motion or framer-motion in package.json, use transition: { type: "spring", duration: 0.3, bounce: 0 } — bounce must always be 0. If no motion library is installed, keep both icons in the DOM (one absolute-positioned) and cross-fade with CSS transitions using cubic-bezier(0.2, 0, 0, 1) — this gives both enter and exit animations without any dependency.

8. Font Smoothing

Apply -webkit-font-smoothing: antialiased to the root layout on macOS for crisper text.

9. Tabular Numbers

Use font-variant-numeric: tabular-nums for any dynamically updating numbers to prevent layout shift.

10. Text Wrapping

Use text-wrap: balance on headings. Use text-wrap: pretty for body text to avoid orphans.

11. Skip Animation on Page Load

Use initial={false} on AnimatePresence to prevent enter animations on first render. Verify it doesn't break intentional entrance animations.

12. Never Use transition: all

Always specify exact properties: transition-property: scale, opacity. Tailwind's transition-transform covers transform, translate, scale, rotate.

13. Use will-change Sparingly

Only for transform, opacity, filter — properties the GPU can composite. Never use will-change: all. Only add when you notice first-frame stutter.

14. Minimum Hit Area

Interactive elements need at least 40×40px hit area. Extend with a pseudo-element if the visible element is smaller. Never let hit areas of two elements overlap.

15. No Layout Shift

Content must never reflow after it appears. Reserve space up front for anything that loads or changes: fix dimensions on images and media (width/height or aspect-ratio), give async content a placeholder or skeleton of the same size, and keep dynamic numbers on tabular-nums. Toggling state (loading, error, expanded) must not resize the surrounding layout, and late-loading fonts must not shift text. Animate only compositor properties (transform, opacity), never geometry like width, height, top, or margin.

16. Canonical Tailwind Classes

On Tailwind, prefer a scale class over an arbitrary value when one exists: size-8.5 over size-[34px], duration-220 over duration-[220ms]. Every px on the spacing scale maps by N / 4. Reserve [...] for values with no canonical form (breakpoints, color-mix, cubic-bezier, rounded-[…]). Bare numeric values like size-8.5 and duration-220 need Tailwind v4, so on v3 check the project's theme scale first.

Common Mistakes

Verifying the rendered result

Whenever a change needs to be seen to be trusted, hand the verification to the cdp skill when it is installed. It captures the page and measures the rendered DOM, and its numbers are the evidence. Without it, measure the rendered DOM by whatever means the project already has, and never settle for a screenshot that merely looks right.

Review Output Format

Always present changes as a markdown table with Before and After columns. Include every change you made — not just a subset. Never list findings as separate "Before:" / "After:" lines outside of a table. Group changes by principle using a heading above each table, and keep each row focused on a single diff so the reader can scan the whole list quickly.

Example

Concentric border radius

Tabular numbers

Rows should cite the specific file and the specific property that changed when it isn't obvious from the snippet. If a principle was reviewed but nothing needed to change, omit that table entirely — empty tables add noise.

Review Checklist

• Nested rounded elements use concentric border radius
• Icons are optically centered, not just geometrically
• Shadows used instead of borders where appropriate
• Enter animations are split and staggered
• Exit animations are subtle
• Dynamic numbers use tabular-nums
• Font smoothing is applied
• Headings use text-wrap: balance
• AnimatePresence uses initial={false} for default-state elements
• No transition: all — only specific properties
• will-change only on transform/opacity/filter, never all
• Interactive elements have at least 40×40px hit area
• No layout shift: space is reserved for loading/async/media content, toggles don't resize the layout
• Scale classes used over arbitrary values where a canonical form exists

Reference Files

• typography.md — Text wrapping, font smoothing, tabular numbers
• surfaces.md — Border radius, optical alignment, shadows
• animations.md — Interruptible animations, enter/exit transitions, icon animations
• performance.md — Transition specificity, will-change usage

Credits

Adapted from make-interfaces-feel-better by Jakub Krehel.