Pen Settings



CSS Base

Vendor Prefixing

Add External Stylesheets/Pens

Any URLs added here will be added as <link>s in order, and before the CSS in the editor. You can use the CSS from another Pen by using its URL and the proper URL extension.

+ add another resource


Babel includes JSX processing.

Add External Scripts/Pens

Any URL's added here will be added as <script>s in order, and run before the JavaScript in the editor. You can use the URL of any other Pen and it will include the JavaScript from that Pen.

+ add another resource


Add Packages

Search for and use JavaScript packages from npm here. By selecting a package, an import statement will be added to the top of the JavaScript editor for this package.


Auto Save

If active, Pens will autosave every 30 seconds after being saved once.

Auto-Updating Preview

If enabled, the preview panel updates automatically as you code. If disabled, use the "Run" button to update.

Format on Save

If enabled, your code will be formatted when you actively save your Pen. Note: your code becomes un-folded during formatting.

Editor Settings

Code Indentation

Want to change your Syntax Highlighting theme, Fonts and more?

Visit your global Editor Settings.


  <li>This is originally in light DOM</li>
  <li>This one, too</li>
  <li>Note the CSS style...</li>

<!-- we will use this in our shadow DOM -->
  <!-- This style is only visible for children INSIDE the shadow DOM -->
  <style>li { color: green; }</style>


                li { color: red; } /* only valid outside the shadow DOM */


                const template = document.querySelector('template').content;

class MyList extends HTMLElement {
  connectedCallback() {
    // Note: "this" is the component instance in the DOM tree
    const shadow = this.attachShadow({mode: 'open'})
    const content = template.cloneNode(true)
// grab the <ul> inside our shadow DOM
    const list = shadow.querySelector('ul');     // we query all <li> elements that are inside our component (i.e. the <li> are the light DOM of our component)
   const items = this.querySelectorAll('li')
   // Now we move each of them into the list INSIDE the shadow DOM by appending them to the list...
   items.forEach(li => list.appendChild(li));

window.customElements.define('my-list', MyList);