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Shadow DOM Styling

Shadow DOM encapsulates styles by default, but provides controlled ways to let users customize the appearance: CSS custom properties and CSS parts.

How Style Crosses the Boundary

Page CSS                   Shadow DOM
┌─────────────────┐       ┌─────────────────┐
│  color: red      │──────│  (inherited!)   │
│  font-size: 16px │──────│  (inherited!)   │
│  --primary: blue │──────│  var(--primary) │
│  h2 { ... }     │──✗──→│  (blocked!)     │
│  .class { ... } │──✗──→│  (blocked!)     │
└─────────────────┘       └─────────────────┘

Inherited properties cross the boundary: color, font-family, font-size, line-height

Non-inherited properties are blocked: background, border, margin, padding

CSS Custom Properties (Variables)

The primary theming mechanism for Web Components. Variables cross the shadow boundary:

class ThemedButton extends HTMLElement {
  constructor() {
    super();
    const shadow = this.attachShadow({ mode: 'open' });
    shadow.innerHTML = `
      <style>
        button {
          background: var(--btn-bg, #6366f1);
          color: var(--btn-color, white);
          border: var(--btn-border, none);
          padding: var(--btn-padding, 8px 16px);
          border-radius: var(--btn-radius, 6px);
          cursor: pointer;
        }
      </style>
      <button><slot></slot></button>
    `;
  }
}

The user can customize it via CSS variables:

themed-button {
  --btn-bg: #22c55e;
  --btn-color: #1e293b;
  --btn-radius: 4px;
}

<style>
themed-button { --btn-bg: #6366f1; --btn-color: white; }
themed-button.warning { --btn-bg: #eab308; --btn-color: #1e293b; }
themed-button.danger { --btn-bg: #ef4444; }
</style>
<themed-button>Primary</themed-button>
<themed-button class='warning'>Warning</themed-button>
<themed-button class='danger'>Danger</themed-button>
<pre id='var-out' style='background:#f1f5f9;padding:12px;border-radius:6px;'></pre>

class ThemedButton extends HTMLElement {
constructor() {
super();
const shadow = this.attachShadow({ mode: 'open' });
shadow.innerHTML = `
<style>
button {
background: var(--btn-bg, #6366f1);
color: var(--btn-color, white);
border: none;
padding: 8px 16px;
border-radius: 6px;
cursor: pointer;
font-size: 1rem;
}
</style>
<button><slot></slot></button>
`;
}
}
customElements.define('themed-button', ThemedButton);

document.getElementById('var-out').textContent =
'Each button reads CSS variables from its context:\\n' +
'  --btn-bg for background\\n' +
'  --btn-color for text color\\n' +
'Warning and danger variants override --btn-bg.';

CSS Parts

CSS ::part() lets you expose specific elements inside shadow DOM for external styling:

class MyCard extends HTMLElement {
  constructor() {
    super();
    const shadow = this.attachShadow({ mode: 'open' });
    shadow.innerHTML = `
      <div class="card">
        <h2 part="title"><slot name="title"></slot></h2>
        <div part="body"><slot></slot></div>
        <div part="footer"><slot name="footer"></slot></div>
      </div>
    `;
  }
}

Users can style the exposed parts:

my-card::part(title) {
  color: #6366f1;
  font-size: 1.5rem;
}

my-card::part(footer) {
  border-top: 1px solid #e2e8f0;
  padding-top: 8px;
}

<style>
custom-card::part(title) { color: #6366f1; font-size: 1.3rem; }
custom-card::part(body) { color: #334155; line-height: 1.5; }
custom-card::part(footer) { border-top: 2px solid #e2e8f0; padding-top: 8px; margin-top: 8px; font-size: 0.8rem; color: #94a3b8; }
</style>
<custom-card>
<span slot='title'>Card Title</span>
<p>This is the card body. The parts are styled from outside the shadow DOM.</p>
<span slot='footer'>Footer content</span>
</custom-card>
<pre id='part-out' style='background:#f1f5f9;padding:12px;border-radius:6px;'></pre>

class CustomCard extends HTMLElement {
constructor() {
super();
const shadow = this.attachShadow({ mode: 'open' });
shadow.innerHTML = `
<div style='border:1px solid #e2e8f0;border-radius:8px;padding:16px;background:white;'>
<div part='title'><slot name='title'></slot></div>
<div part='body'><slot></slot></div>
<div part='footer'><slot name='footer'></slot></div>
</div>
`;
}
}
customElements.define('custom-card', CustomCard);

document.getElementById('part-out').textContent =
'The external CSS styles ::part(title), ::part(body), ::part(footer).\\n' +
'CSS parts are the official API for styling shadow DOM internals.';

Export Parts with exportparts

The exportparts attribute re-exports parts from a nested shadow element:

<my-nested-component exportparts="inner-title: title"></my-nested-component>

my-component::part(title) { /* styled via re-export */ }

:host and :host()

Style the custom element itself from within shadow:

:host {
  display: block;
  border: 1px solid #e2e8f0;
  border-radius: 8px;
}

:host(.warning) {
  border-color: #eab308;
}

:host-context(.dark-theme) {
  background: #1e293b;
  color: white;
}

:host-context()

Style based on a parent’s class:

:host-context(.dark-mode) {
  --text: white;
  --bg: #1e293b;
}

Styling Decision Guide

Key Takeaways

• CSS custom properties are the recommended theming mechanism for Web Components
• ::part() exposes specific elements for external styling
• :host styles the component itself from inside shadow
• Inheritable properties (color, font, etc.) cross the shadow boundary
• :host-context() lets you respond to parent CSS classes
• Avoid relying on global styles — make components self-contained with CSS variables
• Always provide sensible defaults for CSS variables with var(--name, default)