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Shadow DOM

Shadow DOM provides DOM and style encapsulation. Styles from the main page don’t leak into the shadow tree, and shadow styles don’t leak out.

What is Shadow DOM?

Light DOM (main page)
┌────────────────────────────────┐
│  <my-component>                │
│  ┌── Shadow Root ──────────┐   │
│  │  <style> h1 { color:    │   │
│  │    red; } </style>      │   │
│  │  <h1>Shadow DOM</h1>    │   │
│  │  <slot></slot>          │   │
│  └─────────────────────────┘   │
│  </my-component>               │
└────────────────────────────────┘

Creating Shadow DOM

class MyComponent extends HTMLElement {
  constructor() {
    super();
    // Attach shadow root
    const shadow = this.attachShadow({ mode: 'open' });

    // Add content to shadow
    shadow.innerHTML = `
      <style>
        h2 { color: #6366f1; }
        p { font-size: 0.9rem; }
      </style>
      <h2>Shadow Component</h2>
      <p>This is inside the shadow DOM</p>
    `;
  }
}
customElements.define('my-component', MyComponent);

<style>
/* This style tries to style h3 in the component */
h3 { color: #ef4444 !important; font-style: italic; }
</style>
<shadow-demo></shadow-demo>
<pre id='shadow-out' style='background:#f1f5f9;padding:12px;border-radius:6px;'></pre>

class ShadowDemo extends HTMLElement {
constructor() {
super();
const shadow = this.attachShadow({ mode: 'open' });
shadow.innerHTML = `
<style>
h3 { color: #6366f1; font-size: 1.3rem; border-bottom: 2px solid #6366f1; padding-bottom: 4px; }
p { color: #334155; background: #f1f5f9; padding: 8px; border-radius: 4px; }
</style>
<h3>Shadow DOM</h3>
<p>Styled inside shadow — the page's CSS can't reach me!</p>
<p>The red italic style in the global CSS has no effect here.</p>
`;
}
}
customElements.define('shadow-demo', ShadowDemo);

document.getElementById('shadow-out').textContent =
'The <h3> above is styled by Shadow DOM CSS (purple).\\n' +
'The global CSS (red, italic) is blocked by Shadow DOM boundary.\\n' +
'This is style encapsulation in action!';

Shadow Root Modes

Use open unless you have a specific reason to close it (closed mode also breaks some DevTools features).

Shadow DOM vs Light DOM

Querying Inside Shadow DOM

const component = document.querySelector('my-component');
const shadow = component.shadowRoot;

// Find elements inside shadow
const h2 = shadow.querySelector('h2');

// Find all paragraphs in shadow
const paragraphs = shadow.querySelectorAll('p');

// Access shadow host from inside
// this.getRootNode().host returns the custom element

Multiple Shadow Roots

An element can only have one shadow root. Attaching a second throws an error.

Slots in Shadow DOM

Slots let you project light DOM content into shadow DOM:

class FancyBox extends HTMLElement {
  constructor() {
    super();
    const shadow = this.attachShadow({ mode: 'open' });
    shadow.innerHTML = `
      <style>
        .box { border: 2px solid #6366f1; border-radius: 8px; padding: 16px; }
        ::slotted(h2) { color: #6366f1; margin: 0; }
      </style>
      <div class="box">
        <slot name="title">Default Title</slot>
        <slot>Default content</slot>
      </div>
    `;
  }
}

Usage:

<fancy-box>
  <h2 slot="title">My Title</h2>
  <p>This goes into the default slot</p>
</fancy-box>

Shadow DOM Style Encapsulation

• CSS from the page does NOT affect shadow DOM content
• CSS from shadow DOM does NOT affect the page
• Inheritable styles (like font-family and color) do cascade into shadow DOM
• CSS custom properties (variables) do cross the shadow boundary

Practical: Themed Shadow Component

<style>
:root { --primary: #6366f1; --bg: #f8fafc; }
.dark-theme { --primary: #22c55e; --bg: #0f172a; }
</style>
<themed-card>
<span slot='title'>Shadow Card</span>
<p>Content with themed colors via CSS variables</p>
</themed-card>
<button id='toggle-theme' style='margin-top:8px;'>Toggle Dark Theme</button>
<pre id='themed-out' style='background:#f1f5f9;padding:12px;border-radius:6px;'></pre>

class ThemedCard extends HTMLElement {
constructor() {
super();
const shadow = this.attachShadow({ mode: 'open' });
shadow.innerHTML = `
<style>
.card { border: 2px solid var(--primary, #6366f1); border-radius: 8px; padding: 16px; background: var(--bg, white); }
h3 { color: var(--primary, #6366f1); margin: 0 0 8px; }
::slotted(p) { color: #334155; }
</style>
<div class='card'>
<h3><slot name='title'>Card</slot></h3>
<slot></slot>
</div>
`;
}
}
customElements.define('themed-card', ThemedCard);

let dark = false;
document.getElementById('toggle-theme').onclick = function() {
dark = !dark;
document.body.classList.toggle('dark-theme', dark);
document.getElementById('themed-out').textContent = dark ? 'Dark theme applied' : 'Light theme applied';
};

Key Takeaways

• Shadow DOM provides style and DOM encapsulation
• Attach with element.attachShadow({ mode: 'open' })
• Global CSS doesn’t affect shadow DOM (except CSS variables and inherited properties)
• Shadow DOM styles don’t leak out
• Use shadowRoot.querySelector() to find elements inside shadow
• Slots project light DOM content into the shadow tree
• CSS custom properties cross the shadow boundary — use them for theming
• An element can have only one shadow root