interface

name	interface
description	Design engineering principles for making interfaces feel polished. Use when building UI, reviewing frontend code, or working on any visual detail, from animations, hover states, shadows, borders, and typography to optical alignment and tabular numbers.
user-invocable	true
metadata	{"internal":true}

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

Category	When to Use
Typography	Text wrapping, font smoothing, tabular numbers
Surfaces	Border radius, optical alignment, shadows, hit areas
Animations	Interruptible animations, enter/exit transitions, icon animations
Performance	Transition specificity, will-change usage

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

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-[…]).

Common Mistakes

Mistake	Fix
Same border radius on parent and child	Calculate outerRadius = innerRadius + padding
Icons look off-center	Adjust optically with padding or fix SVG directly
Hard borders between sections	Use layered box-shadow with transparency
Jarring enter/exit animations	Split, stagger, and keep exits subtle
Numbers cause layout shift	Apply tabular-nums
Heavy text on macOS	Apply antialiased to root
Animation plays on page load	Add initial={false} to AnimatePresence
transition: all on elements	Specify exact properties
First-frame animation stutter	Add will-change: transform (sparingly)
Tiny hit areas on small controls	Extend with pseudo-element to 40×40px
Content reflows after load or on toggle	Reserve space up front (fixed dimensions, aspect-ratio, skeleton)
Arbitrary value where a scale class fits	Use the canonical class (size-8.5, not size-[34px])

Verifying the rendered result

Verify by measuring, not by eyeballing. For any spacing or alignment claim, measure the rendered DOM (getBoundingClientRect between an element and its container's edges) and report the numbers. A screenshot that looks aligned is not evidence.

For complex visual changes, or whenever seeing the rendered result matters, drive the running page through Chrome's built-in DevTools Protocol (CDP) to inspect the DOM, computed styles, and layout, and to capture screenshots. It works against any URL, whatever app or domain is serving it. The tools/cdp.ts helper does exactly this:

npx tsx tools/cdp.ts --out ./temp/shots --shot 'label|<url>'

Pass an optional --measure snippet to read geometry off the rendered DOM. Put any query params the page supports (a forced theme, a route, a flag) straight in the URL. Reach for it when a change is hard to verify from the source alone: layout, spacing, alignment, overflow, theme tokens, or anything that only shows up once rendered. Skip it for simple, self-evident edits (for example, adding a shadow) where the result is obvious from the diff.

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

Before	After
rounded-xl on card + rounded-xl on inner button (p-2)	rounded-2xl on card (12 + 8), rounded-lg on inner button
border-radius: 16px on both nested surfaces	Outer 24px, inner 16px with 8px padding

Tabular numbers

Before	After
<span>{count}</span> on animated counter	<span className="tabular-nums">{count}</span>
Default numerals on timer	Added font-variant-numeric: tabular-nums to root

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.