Native Web Components Skill

What is it?

A skill for building framework-free web components using only native HTML, CSS, and modern browser platform features. It prioritizes Declarative Shadow DOM (DSD), CSS-first interactivity, and the newest baseline features (2025 → 2024 → 2023) to ensure compatibility across the latest four stable browser versions.

Why use it?

• Zero dependencies: No frameworks, bundlers, or build tools required
• Maximum performance: Native platform features are faster than JavaScript alternatives
• Cross-browser stability: Baseline features ensure support across Chrome, Edge, Firefox, Safari
• Future-proof: Uses the newest standards with graceful degradation
• Minimal JavaScript: CSS handles interactivity; JS only for progressive enhancement

How to use it?

Workflow (feature-first)

1. Select the newest baseline feature that fits (2025 → 2024 → 2023). Avoid non-baseline features.
2. Start with semantic HTML in light DOM, then add DSD via <template shadowrootmode="open">.
3. Use CSS-first interactivity and native UI primitives (details, popover, dialog, :has()).
4. Make layout responsive with container queries and responsive units; avoid px for layout.
5. Add JavaScript only for behavior HTML/CSS cannot express; register custom elements only when lifecycle hooks are required.
6. Validate with the checklist and test on Chrome, Edge, Firefox, Safari latest stable.

Component skeleton (DSD-first)

<my-card>
  <template shadowrootmode="open">
    <style>
      :host {
        display: block;
        container-type: inline-size;
        padding: 1rem;
      }
    </style>
    <slot></slot>
  </template>
  <h2>Card title</h2>
</my-card>

Mandatory rules

• Use DSD only; never call attachShadow().
• Avoid frameworks or component libraries (Lit, React, Vue).
• Avoid JavaScript by default; treat JS as progressive enhancement only.
• Use responsive units (rem, em, %, cqi, dvh, clamp()); avoid px for layout.
• Use container queries for component layout; media queries only for page layout.
• Use native overlays (popover, dialog) instead of custom modals.
• Use @scope and @layer to prevent style leakage.
• Follow the 2025 -> 2024 -> 2023 feature ladder.
• Follow the project class naming convention (TBD); do not invent one here.

Feature ladder (summary)

• 2025: @scope, ::details-content, view-transition-class, scrollend, content-visibility
• 2024: DSD, Popover, @starting-style, transition-behavior, light-dark(), text-wrap: balance, @property
• 2023: container queries/units, :has(), subgrid

<!-- NOTE: Anchor positioning is NOT yet Baseline (limited status) -->

Use the full matrix in references/baseline-features.md.

Minimal JavaScript policy

• Allow: custom element registration, event bridging, attribute reflection, optional dialog helpers.
• Avoid: rendering UI, templating, state machines, or framework APIs.
• Prefer: HTML primitives and CSS features.

Folder structure (skill layout)

instructions/skills/dev-native-web-components/
  SKILL.md
  references/
    baseline-features.md
    declarative-shadow-dom.md
    slot-patterns.md
    forms.md
    responsive-design.md
    css-first-patterns.md
    popover-dialog.md
    view-transitions.md
    accessibility.md
    performance.md
    debugging.md
    validation-checklist.md
    css.md
    html.md
  examples/
    card-component/
    popover-menu/
    responsive-grid/
    accordion/
    scope-details/
    view-transitions/
    has-subgrid/

Examples

• card-component/ - DSD card with container queries
• popover-menu/ - Popover navigation menu
• responsive-grid/ - Container query grid layout
• accordion/ - Details/summary accordion
• scope-details/ - @scope and ::details-content FAQ
• view-transitions/ - Minimal view transition toggle
• has-subgrid/ - :has() + subgrid spec table

Testing & Debugging

• E2E Testing: Use the playwright-testing skill for component testing. See [debugging.md](references/debugging.md).
• Debugging: Use Playwright MCP Server or native browser DevTools. See [debugging.md](references/debugging.md).

Supporting References

Limitations

• JavaScript patterns (minimal-javascript.md) will be added later
• Design tokens strategy will be added later
• Multi-component communication requires JS — on hold
• Fallback patterns for non-baseline features require JS — on hold