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, image outlines, hit areas
Animations	Interruptible animations, enter/exit transitions, icon animations, scale on press
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

For elevation (dropdowns, modals, cards) use the shadow-subtle/shadow-medium/shadow-overlay/shadow-card tokens. In this repo neutral edges and dividers stay as --border borders (.claude/rules/sim-styling.md, Line weight) — do not swap them for 0 0 0 1px shadow rings.

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 contextual icons with opacity, scale, and blur instead of toggling visibility; see animations.md for the Motion and CSS cross-fade patterns.

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. Image Outlines

Add a subtle 1px low-opacity outline to images (outline-black/10 light, outline-white/10 dark); see surfaces.md.

12. Scale on Press

A subtle scale-down (about 0.96-0.97) on press gives tactile feedback. In this repo a press affordance belongs in the emcn Button/Chip chrome (packages/emcn), not in consumer classes — neither component implements one today, so propose it there rather than adding per-call-site transforms.

13. 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.

14. Never Use transition: all

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

15. 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.

16. 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.

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	In this repo, the --border hairline token; elsewhere, 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

Review Output Format

Present changes as markdown tables with Before and After columns, one table per principle with a heading above it, one diff per row, and cite file and property when the snippet is not self-explanatory.

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

Scale on press

Before	After
<button className="...">	Added active:scale-[0.96] transition-transform
scale(0.9) on press	Raised to scale(0.96) — anything below 0.95 feels exaggerated

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
• Images have subtle outlines
• Buttons use scale on press where appropriate
• 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

Reference Files

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