design-prototyping - SKILL.md Agent Skill

name: design-prototyping description: The craft of building design exploration prototypes. Covers file structure, control wiring, styling conventions, and output validation. Preloaded into the html-prototyper agent. Not intended for direct invocation. user-invocable: false disable-model-invocation: false

Design Prototyping

How to build a single self-contained design exploration variation file.

File Format

Every variation is a single HTML file with embedded metadata. The file is a complete page rendered inside an iframe in the gallery shell.

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <script src="https://cdn.tailwindcss.com"></script>
  <link rel="preconnect" href="https://fonts.googleapis.com">
  <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
  <!-- Google Fonts <link> for specified fonts -->
  <script type="application/json" id="variation-meta">
  {
    "id": "A1",
    "family": "A",
    "familyName": "...",
    "name": "...",
    "layoutType": "...",
    "aesthetic": "...",
    "description": "...",
    "controls": [ /* control objects — see Control Schema below */ ]
  }
  </script>
  <style>
    :root {
      /* All theme values as CSS custom properties */
    }
    :root * {
      transition: color 0.3s ease, background-color 0.3s ease,
                  border-color 0.3s ease, padding 0.3s ease,
                  gap 0.3s ease, font-size 0.3s ease,
                  border-radius 0.3s ease, box-shadow 0.3s ease,
                  max-width 0.3s ease, width 0.3s ease;
    }
  </style>
</head>
<body>
  <!-- Variation content -->
</body>
</html>

Metadata Block

The <script type="application/json" id="variation-meta"> block in <head> contains valid JSON with:

Field	Type	Description
`id`	string	Variation code, e.g. `"A1"`, `"B2"`
`family`	string	Family letter, e.g. `"A"`
`familyName`	string	Human-readable family name
`name`	string	Memorable variation name
`layoutType`	string	Layout description, e.g. `"Sidebar + Cards"`
`aesthetic`	string	Aesthetic description, e.g. `"Clean Light"`
`description`	string	One-line description of the variation
`controls`	array	4-6 control objects (see schema below)

Populate all fields from the variation brief provided in the prompt.

Styling Rules

Tailwind-first. Utility classes for layout, spacing, colors, typography, borders, shadows. Custom CSS in <style> only for: :root property definitions, @keyframes, transitions, scrollbar styling.
CSS custom properties on :root. All theme values: colors, fonts, spacing, radii. Unprefixed names: --bg, --text, --accent. Never --shell-*. Reference via Tailwind arbitrary values: bg-[var(--bg)], text-[var(--text)], rounded-[var(--radius)].
Self-contained. No external images. Use CSS gradients, inline SVG, Unicode.
Realistic content only. Never lorem ipsum. Use the data from the prompt.
Motion. Hover transitions on interactive elements. At least one entrance animation with staggered delays. JavaScript is allowed for interactive demonstrations (accordions, toggles, wizards).
Component scope. For components (not full pages), center content in the viewport: min-h-screen flex items-center justify-center p-8 on <body>.
Viewport containment. The outermost container element must use overflow-hidden (or overflow-auto if scrolling is intentional). This prevents content from breaking out of the iframe bounds. Apply to the root layout wrapper, not <body>.
Transition CSS required. The :root * { transition: ... } rule (shown in file format above) must be included for smooth control changes.

Control Schema

Controls are JSON metadata only. Define them in the controls array inside the variation-meta block. A separate gallery shell reads this JSON and renders the control UI (sliders, dropdowns, toggles) outside the iframe.

Do NOT build any control panel, settings panel, or configuration UI into the variation HTML. The variation contains only the design content. All control rendering and interaction is handled by the shell.

Include 4-6 controls in the controls array.

Two types of controls

CSS controls (default) — The shell sets a CSS custom property on :root via style.setProperty(). The HTML references it via var(). This is automatic — no JS needed in the variation. Use for visual parameters: colors, spacing, radii, opacity, font sizes, layout widths.

Event controls — For behavioral parameters that CSS can't express (sort order, filter thresholds, data grouping, expansion mode). The shell sets the CSS var AND dispatches a CustomEvent on the iframe's document. The variation includes a JS listener that reads the value and updates the DOM.

To make a control an event control, add "event": true to the control JSON. The shell dispatches control-change events with { detail: { id, value } } for all controls, but only event controls need a listener.

Event controls still need a cssVar (it can be a dummy like "--sort-order") so the shell has something to set. The actual work happens in your listener.

Range

Value + unit are set directly on the CSS var.

{
  "id": "sidebar-width",
  "label": "Sidebar Width",
  "type": "range",
  "min": 180, "max": 320, "step": 10,
  "options": null,
  "value": 240,
  "defaultValue": 240,
  "unit": "px",
  "cssVar": "--sidebar-width"
}

Select (single CSS var)

Maps option labels to CSS values via cssValues.

{
  "id": "accent",
  "label": "Accent Color",
  "type": "select",
  "min": null, "max": null, "step": null,
  "options": ["coral", "teal", "indigo", "amber"],
  "cssValues": { "coral": "#e07a5f", "teal": "#4a9e8f", "indigo": "#5c6bc0", "amber": "#d4a853" },
  "value": "teal",
  "defaultValue": "teal",
  "unit": "",
  "cssVar": "--accent"
}

Select (multi-var)

When one control changes multiple CSS properties, use an object as the cssValues value and set cssVar: null.

{
  "id": "mood",
  "label": "Mood",
  "type": "select",
  "min": null, "max": null, "step": null,
  "options": ["light", "dark", "midnight"],
  "cssValues": {
    "light":    { "--bg": "#faf9f7", "--text": "#2d2a26", "--text-dim": "#8a8580", "--border": "#e5e0d8" },
    "dark":     { "--bg": "#1e1e2a", "--text": "#e0ddd8", "--text-dim": "#8a8580", "--border": "#333340" },
    "midnight": { "--bg": "#0d0d14", "--text": "#c8c4be", "--text-dim": "#6a6660", "--border": "#1e1e2a" }
  },
  "value": "light",
  "defaultValue": "light",
  "unit": "",
  "cssVar": null
}

Toggle

Maps true/false to CSS values. Without cssValues, defaults to "1"/"0".

{
  "id": "show-dividers",
  "label": "Show Dividers",
  "type": "toggle",
  "min": null, "max": null, "step": null,
  "options": null,
  "cssValues": { "true": "1px", "false": "0px" },
  "value": true,
  "defaultValue": true,
  "unit": "",
  "cssVar": "--divider-width"
}

Event Control Pattern

When a control needs to drive behavior (not just CSS), mark it with "event": true and add a listener in the variation's <script>:

{
  "id": "sort-order",
  "label": "Sort By",
  "type": "select",
  "options": ["status", "last-active", "name"],
  "cssValues": { "status": "status", "last-active": "last-active", "name": "name" },
  "value": "status",
  "defaultValue": "status",
  "unit": "",
  "cssVar": "--sort-order",
  "event": true
}

<script>
  document.addEventListener('control-change', (e) => {
    const { id, value } = e.detail;
    if (id === 'sort-order') {
      sortAgents(value);  // your function that re-sorts the DOM
    }
  });

  // Also handle initial state — controls-ready fires once after all
  // CSS vars are set on first iframe load
  document.addEventListener('controls-ready', () => {
    const sort = getComputedStyle(document.documentElement)
      .getPropertyValue('--sort-order').trim();
    if (sort) sortAgents(sort);
  });
</script>

The controls-ready event fires once after all controls are applied on iframe load. Use it to read initial CSS var values and set up initial state. Individual control-change events fire on each subsequent control adjustment.

Control Rules

id is required. Unique within the variation. Without it, the control renders but interactions do nothing.
value and defaultValue must always be set. Never null or undefined.
cssValues must be an object keyed by option labels, never an array. { "compact": "0.75rem" } not ["0.75rem", "1rem"].
Never use unit: "%" on range controls. The % breaks calc() expressions. Use unitless values and let CSS handle the math: calc(var(--opacity) * 1%).
Select controls must include cssValues. Without it, the CSS var gets a label string like --bg: warm which is useless to CSS.
Every cssVar must be consumed in the HTML. Reference via Tailwind arbitrary values (rounded-[var(--card-radius)]) or in CSS. A control that defines --card-radius is dead if the HTML uses rounded-xl instead.
All visual states must use var() for controlled properties. Hover, active, and focus states must reference CSS vars, not hardcoded values. Use color-mix() for derived colors: color-mix(in srgb, var(--accent) 10%, transparent).
Never use [style*="..."] attribute selectors for CSS variable detection. They are fragile.
CSS custom properties on :root only. Not scoped by class. The iframe provides isolation. Controls apply via documentElement.style.setProperty().
The control system can only set CSS custom properties. It cannot toggle CSS classes. All controlled styling must flow through var() references.
Behavioral controls need "event": true and a JS listener. If a control drives behavior (sorting, filtering, thresholds, expansion mode), mark it "event": true and add a control-change event listener. Without the listener, the CSS var is set but nothing reads it.

CRITICAL: The metadata block MUST use exactly id="variation-meta". This ID is machine-parsed by the assembly script. Not variation-metadata, not exploration-metadata — exactly variation-meta. If the ID is wrong, the variation will silently fail to appear in the gallery.

Pre-Output Checklist

Verify before writing:

<script type="application/json" id="variation-meta"> block is in <head> with valid JSON
Every control has an id field (unique within this variation)
Every control's cssVar appears as var(--xxx) in the HTML or is consumed by a control-change event listener
cssValues is an object keyed by option labels, never an array
All controlled styling uses var() references, no CSS class toggling
No range control uses unit: "%"
Hover/active/focus states use var() for any property driven by a control
Every <template> open tag has a matching </template> close tag (balanced nesting is safe; orphaned close tags break assembly)
CSS custom properties are defined on :root (not scoped by class)
HTML includes Tailwind CDN and Google Fonts in <head>
HTML includes transition CSS for smooth control changes
HTML contains NO control panel UI (no sliders, dropdowns, toggles, settings panels)
The outermost container uses overflow-hidden or overflow-auto

Mandatory QA — Controls Audit

After writing the HTML, before finishing, audit every control against its wiring:

For each control in the controls array:

Find where it's consumed. Search the HTML for var(--{cssVar}) or the control-change listener handling its id.
If not consumed, fix it. Either: (a) add the var() reference to the HTML where the property should apply, or (b) add a control-change listener if the control is behavioral.
Verify visible impact. Ask: "If I move this slider from min to max (or toggle this), would the change be obvious to someone across the room?" If not, widen the range or choose a more impactful property.

This audit catches the most common failure mode: controls that are defined in JSON but have no effect on the rendered output.

Forbidden

No control UI in the variation. Never build sliders, dropdowns, toggle switches, settings panels, or any control interface into the HTML. Controls are JSON metadata; the gallery shell renders all control UI.
No orphaned </template> tags. Every </template> must have a matching <template> open tag. The assembly system wraps each file in a <template> element; an orphaned close tag would break the wrapper. Balanced <template> nesting (e.g., Alpine.js <template x-for>, <template x-if>) is safe.