Pen Settings



CSS Base

Vendor Prefixing

Add External Stylesheets/Pens

Any URL's added here will be added as <link>s in order, and before the CSS in the editor. If you link to another Pen, it will include the CSS from that Pen. If the preprocessor matches, it will attempt to combine them before processing.

+ add another resource


Babel is required to process package imports. If you need a different preprocessor remove all packages first.

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


Save Automatically?

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.


  <toggle-section open="false">
    <h2>Section 1</h2>
    <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nam non lectus sit amet nunc facilisis molestie. Praesent quis libero et mauris facilisis dignissim at sed nisi.</p> 
    <p>Nullam efficitur porttitor lectus, ac finibus nibh fermentum ac. Phasellus aliquam, nibh non efficitur pharetra, tellus diam posuere lectus, a consequat elit ex nec ligula.</p>
  <toggle-section open="true">
    <h2>Section 2</h2>
    <p>Aliquam erat volutpat. Nulla facilisi. Nunc porttitor, elit non eleifend aliquam, est leo scelerisque nibh, nec faucibus odio urna ac nulla.</p> 
    <p>Maecenas laoreet in metus eget convallis. Vivamus at eleifend felis. Proin non vehicula neque. Etiam eleifend sapien ut nulla malesuada, ac condimentum nisl efficitur.</p>



                html {
  font-family: Arial, sans-serif;

body {
  max-width: 40rem;
  margin: 0 auto;
  padding: 1em;

.controls {
 text-align: right;
 margin-bottom: 1em;

.controls li {
  display: inline;

button {
  background: #000;
  color: #fff;
  border: 0;
  font-size: 0.85rem;
  border-radius: 0.25rem;

Custom elements are inline by default
toggle-section {
  display: block;

Only applies if script runs and 
`role="region"` is added
toggle-section[role="region"] {
  border-width: 2px 0;
  border-style: solid;

toggle-section[role="region"] + toggle-section {
  border-top: 0;


                (function() {
  // Check for <template> support
  if ('content' in document.createElement('template')) {
    const tmpl = document.createElement('template')

    // Create the web component's template
    // featuring a <slot> for the Light DOM content
    tmpl.innerHTML = `
        <button aria-expanded="false">
          <svg aria-hidden="true" focusable="false" viewBox="0 0 10 10">
            <rect class="vert" height="8" width="2" y="1" x="4"/>
            <rect height="2" width="8" y="4" x="1"/>
      <div class="content" hidden>
        h2 {
          margin: 0;

        h2 button {
          all: inherit;
          box-sizing: border-box;
          display: flex;
          justify-content: space-between;
          width: 100%;
          padding: 0.5em 0;

        h2 button:focus svg {
          outline: 2px solid;

        button svg {
          height: 1em;
          margin-left: 0.5em;

        [aria-expanded="true"] .vert {
          display: none;

        [aria-expanded] rect {
          fill: currentColor;
    // Check for latest Shadow DOM syntax support
    if (document.head.attachShadow) {
      class ToggleSection extends HTMLElement {
        constructor() {

          // Make the host element a region
          this.setAttribute('role', 'region')

          // Create a `shadowRoot` and populate from template 
          this.attachShadow({ mode: 'open' })

          // Assign the toggle button
          this.btn = this.shadowRoot.querySelector('h2 button')

          // Get the first element in Light DOM
          // and cast its heading level (which should, but may not, exist)
          const oldHeading = this.querySelector(':first-child')
          let level = parseInt(oldHeading.tagName.substr(1))

          // Get the Shadow DOM <h2>
          this.heading = this.shadowRoot.querySelector('h2')
           // If there is no level, there is no heading.
          // Add a warning.
          if (!level) {
            console.warn('The first element inside each <toggle-section> should be a heading of an appropriate level.')
          // If the level is a real integer and not 2
          // set `aria-level` accordingly
          if (level && level !== 2) {
            this.heading.setAttribute('aria-level', level)

          // Add the Light DOM heading label to the innerHTML of the toggle button
          // and remove the now unwanted Light DOM heading
          this.btn.innerHTML = oldHeading.textContent + this.btn.innerHTML

          // The main state switching function
          this.switchState = () => {
            let expanded = this.getAttribute('open') === 'true' || false

            // Toggle `aria-expanded`
            this.btn.setAttribute('aria-expanded', expanded)
            // Toggle the `.content` element's visibility
            this.shadowRoot.querySelector('.content').hidden = !expanded

          // Change the component's `open` attribute value on click
          // (which will, in turn, trigger switchState(), see below)
          this.btn.onclick = () => { 
            this.setAttribute('open', this.getAttribute('open') === 'true' ? 'false' : 'true')

        // Identify just the `open` attribute as an observed attribute
        static get observedAttributes() {
          return ['open']

        // When `open` changes value, execute switchState()
        attributeChangedCallback(name) {
          if (name === 'open') {

      // Add our new custom element to the window for use
      window.customElements.define('toggle-section', ToggleSection) 
      // Define the expand/collapse all template
      const buttons = document.createElement('div')
      buttons.innerHTML = `
        <ul class="controls" aria-label="section controls">
          <li><button id="expand">expand all</button></li>
          <li><button id="collapse">collapse all</button></li>

      // Get the first `toggle-section` on the page
      // and all toggle sections as a node list
      const first = document.querySelector('toggle-section')
      const all = document.querySelectorAll('toggle-section')

      // Insert the button controls before the first <toggle-section>
      first.parentNode.insertBefore(buttons, first)

      // Place the click on the parent <ul>...
      buttons.addEventListener('click', (e) => {
        // ...then determine which button was the target 
        let expand = === 'expand' ? true : false

        // Iterate over the toggle sections to switch
        // each one's state uniformly, (t) => {
          t.setAttribute('open', expand)