Design Reverse Engineer

Extract a design system from any URL and produce a reusable spec file for your project.

When to use

• Starting a new view/component and want to anchor it in proven design rather than generic output
• User points at a site and says "make it look like this"
• Refreshing an existing view — need a reference to measure against
• Exploring what works before writing any code

When NOT to use

• You already have a design direction — don't reverse-engineer on a whim
• The target site is login-gated and you don't have credentials (capture screenshots via /webapp-testing instead)
• The target uses heavy runtime JS that WebFetch can't see through (use Playwright ultra-mode instead)
• Simple one-off style tweaks — use /frontend-design token lookup directly

Workflow

Step 1 — Fetch the URL

Two modes depending on complexity:

Standard mode (WebFetch): Good for static/SSR sites with CSS in <style> blocks or linked sheets.

WebFetch url="<target>" prompt="Extract all CSS custom properties, font stacks, color values, spacing patterns, and component class names visible in the HTML."

Follow up with:

WebFetch url="<target>/about" or "/pricing" prompt="Same extraction — compare component variation."

Ultra mode (Playwright via /webapp-testing): Required when you need hover states, focus rings, animated transitions, or sites with heavy client-side rendering.

Steps for ultra mode:

1. Open target in Chrome via mcp__chrome-devtools__navigate_page
2. mcp__chrome-devtools__take_screenshot — initial state
3. mcp__chrome-devtools__hover over key interactive elements (buttons, nav, cards)
4. Screenshot after each hover
5. mcp__chrome-devtools__evaluate_script to dump getComputedStyle for target elements:

   const el = document.querySelector('.btn-primary');
   const s = getComputedStyle(el);
   JSON.stringify({ bg: s.backgroundColor, color: s.color, radius: s.borderRadius, font: s.fontFamily });

6. Dump all CSS custom properties from :root:

   const props = {};
   for (const sheet of document.styleSheets) {
     try {
       for (const rule of sheet.cssRules) {
         if (rule.selectorText === ':root') {
           for (const prop of rule.style) props[prop] = rule.style.getPropertyValue(prop).trim();
         }
       }
     } catch(e) {}
   }
   JSON.stringify(props, null, 2);

Step 2 — Extract design tokens

Identify and record:

Step 3 — Identify component patterns

For each major component type found:

• Buttons: background, border, text color, radius, hover delta, focus ring, disabled state
• Cards: background, border, shadow, padding, radius, hover lift or highlight
• Tables: header background, row hover, border style (full grid vs horizontal-only), cell padding
• Badges/Tags: background, text, radius, variants (success/warning/error)
• Forms/Inputs: border, focus ring, placeholder color, error state
• Empty states: illustration vs text-only, CTA placement
• Loading states: skeleton vs spinner vs shimmer pattern
• Modals/Dialogs: backdrop, surface, padding, header treatment

Step 4 — Capture micro-interactions (ultra mode only)

Document:

• Hover: background delta, transform (lift? scale?), transition duration
• Focus: ring color, ring offset, outline vs box-shadow approach
• Active/pressed: scale-down, brightness change
• Disabled: opacity level, cursor
• Loading: spinner position, text swap, skeleton pattern

Step 5 — Write the spec

Save to .claude/design-references/<slug>.md (create dir if needed). Slug = kebab-case site name, e.g. linear-app.md, stripe-dashboard.md.

Output format

The spec file this skill produces:

# <Site Name> Design Reference

Source: <url>
Captured: <YYYY-MM-DD>
Mode: WebFetch | Playwright ultra

## Typography
- Display: <font, weight, size>
- Body: <font, size, line-height>
- Mono/Code: <font>
- Scale: <list of sizes used>

## Color palette

| Token       | Value   | Usage                     |
|-------------|---------|---------------------------|
| bg-app      | #...    | Page background           |
| bg-surface  | #...    | Card/panel backgrounds    |
| bg-elevated | #...    | Hover states              |
| text-primary| #...    | Main content              |
| text-muted  | #...    | Labels, timestamps        |
| accent      | #...    | CTAs, active states       |
| border      | #...    | Standard borders          |

## Spacing

Base unit: 4px / 8px.
Common values: 4, 8, 12, 16, 24, 32, 48, 64 (in px).
Grid columns: N, gap: Xpx.

## Radius

| Element | Value |
|---------|-------|
| Button  | ...   |
| Card    | ...   |
| Input   | ...   |
| Badge   | ...   |

## Components

### Button
- Default: background `#...`, text `#...`, radius `...`
- Hover: background delta `...`, transition `...ms ease`
- Focus: ring `...`
- Disabled: opacity `...`
- (Copy any exposed CSS verbatim here)

### Card
...

### Table
...

### Badge / Tag
...

## Micro-interactions (Playwright ultra only)

| Interaction | Element | Duration | Easing | Effect |
|-------------|---------|----------|--------|--------|
| hover       | .btn    | 150ms    | ease   | bg lighten 10% |
| ...         | ...     | ...      | ...    | ...    |

## What makes this design work

(2-4 sentences: the design philosophy, what creates the cohesion, what's distinctive)

## How to apply this reference

(Which views or components this reference informs, and what specific patterns to borrow)
- Data table → ...
- Info modal → ...
- Form inputs → ...

Practical tips

• Prioritize CSS custom properties — sites that use them expose the entire token system in one getComputedStyle dump
• Capture dark mode separately — prefers-color-scheme: dark may reveal a different palette; toggle via mcp__chrome-devtools__evaluate_script: document.documentElement.setAttribute('data-theme', 'dark')
• Font stack gotchas — getComputedStyle returns the resolved font, not the CSS declaration. Check <link rel=stylesheet> for Google Fonts or @font-face declarations
• Spacing rhythm — look at the inspector's computed margin/padding on a few sibling elements; if they're multiples of 4 or 8 it confirms the base unit
• Don't copy brand colors — extract the palette structure (how many neutrals, how the accent is used) rather than the exact hex values

Cross-reference

• /design-showcase — turn a reference into variant mockups for side-by-side comparison
• /frontend-design — your project's design system tokens; map extracted tokens to your equivalents
• /webapp-testing — Playwright infrastructure for ultra-mode capture
• reference-sites.md (this directory) — curated list of high-quality design references by domain