Custom Elements Skill

Custom Elements Skill

This skill provides guidance for defining and using custom HTML elements in this project.

Two Definition Systems

Both are recommended - elements.json for build-time HTML validation, CEM for runtime tooling.

See MANIFEST.md for Custom Elements Manifest generation.

Using Existing Elements

Check .claude/​schemas/​elements.json for defined custom elements:

Usage Examples

<!-- Product card with attributes -->
<product-card sku="ABC123" price="29.99">
  <h2>Product Name</​h2>
  <p>Product description here.</​p>
</​product-card>

<!-- Void element (self-closing) -->
<icon-element name="star"/>
<user-avatar src="/​avatar.jpg" alt="John Doe" size="medium"/>

<!-- Inline element -->
<p>Status: <status-badge type="success">Active</​status-badge></​p>

<!-- Block elements -->
<nav-menu orientation="horizontal">
  <ul>
    <li><a href="/">Home</​a></​li>
  </​ul>
</​nav-menu>

<data-table source="/​api/​users" sortable="">
  <!-- Table content -->
</​data-table>

Ad-hoc Custom Elements

For one-off custom elements not worth defining in .claude/​schemas/​elements.json, use the x-* prefix:

<x-highlight>Important text</​x-highlight>
<x-tooltip data-text="Help text">Hover me</​x-tooltip>
<x-badge>New</​x-badge>

The x-* pattern is excluded from validation by default.

Defining New Elements

Using Slash Command

/​add-element my-widget

Manual Definition

Add to .claude/​schemas/​elements.json:

{
  "my-element": {
    "flow": true,
    "phrasing": false,
    "permittedContent": ["@flow"],
    "attributes": {
      "required-attr": { "required": true },
      "optional-attr": { "required": false }
    }
  }
}

Element Schema Reference

Content Model

Permitted Content Values

• @flow - Flow content (most elements)
• @phrasing - Phrasing content (inline elements)
• @interactive - Interactive elements
• ["p", "div"] - Specific elements only

Attribute Definitions

{
  "attributes": {
    "name": {
      "required": true           // Must be present
    },
    "type": {
      "required": false,
      "enum": ["a", "b", "c"]    // Restricted values
    },
    "enabled": {
      "boolean": true            // Boolean attribute
    }
  }
}

Complete Examples

Block Element

{
  "card-component": {
    "flow": true,
    "phrasing": false,
    "permittedContent": ["@flow"],
    "attributes": {
      "variant": {
        "required": false,
        "enum": ["default", "outlined", "elevated"]
      },
      "clickable": {
        "boolean": true
      }
    }
  }
}

Usage:

<card-component variant="elevated" clickable="">
  <h2>Card Title</​h2>
  <p>Card content</​p>
</​card-component>

Void Element

{
  "loading-spinner": {
    "void": true,
    "flow": true,
    "phrasing": true,
    "attributes": {
      "size": {
        "enum": ["small", "medium", "large"]
      }
    }
  }
}

Usage:

<loading-spinner size="medium"/>

Inline Element

{
  "price-tag": {
    "flow": true,
    "phrasing": true,
    "permittedContent": ["@phrasing"],
    "attributes": {
      "currency": {
        "required": false,
        "enum": ["USD", "EUR", "GBP"]
      }
    }
  }
}

Usage:

<p>Price: <price-tag currency="USD">29.99</​price-tag></​p>

Naming Conventions

• Use lowercase with hyphens: my-element
• Names must contain a hyphen (web component spec)
• Be descriptive: product-card not pc
• Use consistent prefixes for related elements: form-input, form-select, form-button

---

CSS-Only vs Full Web Components

Custom elements can be used in two distinct ways. Choose based on your needs.

CSS-Only Custom Elements

Use the element as a semantic styling hook without JavaScript:

<product-card>
  <img src="product.jpg" alt="Widget Pro" />
  <h3>Widget Pro</​h3>
  <p>The best widget money can buy.</​p>
</​product-card>

product-card {
  display: block;  /​* Required: browsers default to inline */
  padding: var(--spacing-lg);
  border: 1px solid var(--border);
  border-radius: var(--radius-lg);
}

Characteristics:

• No JavaScript required
• No customElements.define() call
• Matches :not(:defined) pseudo-class
• Requires explicit display: block (handled by shared reset)
• Content is in light DOM (no Shadow DOM)
• Styles work immediately, no FOUC

Best for:

• Semantic styling hooks (replacing .card classes)
• Progressive enhancement base
• Static content components
• Simple layout containers

Full Web Components

Register the element with JavaScript for encapsulated behavior:

class ProductCard extends HTMLElement {
  constructor() {
    super();
    this.attachShadow({ mode: 'open' });
  }

  connectedCallback() {
    this.shadowRoot.innerHTML = `
      <style>
        :host { display: block; }
        /​* Encapsulated styles */
      </​style>
      <slot></​slot>
    `;
  }
}

customElements.define('product-card', ProductCard);

Characteristics:

• Requires JavaScript
• Registered via customElements.define()
• Matches :defined pseudo-class after registration
• Can use Shadow DOM for style encapsulation
• Can have custom properties, methods, lifecycle hooks

Best for:

• Interactive components requiring JS
• Reusable across different projects
• Components needing encapsulated styles
• Complex state management

When to Choose Each

Styling Considerations

CSS-only elements need explicit display:

The shared reset (_reset.css) handles this automatically with:

:not(:defined) {
  display: block;
}

If you need a different display value, override it in your component CSS:

product-card {
  display: grid;  /​* Overrides the reset's block */
}

Web Components control their own display:

// Inside the component
this.shadowRoot.innerHTML = `
  <style>
    :host {
      display: block;
      /​* Component defines its own display */
    }
  </​style>
  ...
`;

The x-* Pattern for Ad-Hoc Elements

For one-off custom elements, use the x-* prefix (e.g., <x-highlight>). These:

• Are excluded from HTML validation by default
• Work well as CSS-only elements
• Signal "ad-hoc, not reusable" to other developers

<p>This is <x-highlight>important text</​x-highlight> in a paragraph.</​p>

x-highlight {
  background: var(--warning-light);
  padding-inline: var(--spacing-xs);
}

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