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HTML

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CSS

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CSS Base

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JavaScript

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Babel includes JSX processing.

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HTML 

              
                <details open>
  <summary>Have you heard about <code>details</code>?</summary>
  <p>It's a pretty useful element that handles accordion / collapsed text natively!</p>
</details>

<details>
  <summary>So how does it work?</summary>
  <p>You wrap a <code>details</code> element around any block of HTML content. The browser will collapse that block of text until a user opens the <code>details</code> block.</p>
  <p>Once a user opens a <code>details</code> block, they'll be able to read all that hidden content!</p>
  <p>If you want the details block open by default, include the <code>open</code> attribute on the opening tag:</p>
  <pre><code><​details open>...<​/details></code></pre>
</details>

<details>
  <summary>But how do I set a custom title?</summary>
  <p>That's pretty manageable too! Use the <code>summary</code> element.</p>
  <p>Put a <code>summary</code> at the beginning of your details element and <b>Boom!</b> - you've got a custom title for your <code>details</code> block.</p>
  <p>No worries if you don't add a <code>summary</code>. The browser will put the word "Details" in there for you. (After all, users need something to click!)</p>
</details>

<details>
  <summary>That's cool, but what about styles?</summary>
  <p>Yes, you're covered there too! Style the <code>details</code> element however you like. Give it a border, some padding, whatever.</p>
  <p>The <code>summary</code> element is where the <code>▸</code> marker lives. If you want to get rid of that, there is a prefixed pseudo-element selector <code>::-webkit-details-marker</code>: set that to <code>display: none</code> for WebKit browsers. In Firefox, it's much simpler: set the <code>summary</code> to <code>display: block</code> or <code>flex</code> (anything but the native <code>display: list-item</code>) and you'll get rid of the <code>▸</code> for you.</p>
</details>

<details>
  <summary>Ok, ok, but what about styling based on state?</summary>
  <p>Once again, <code>details</code> has got your back! When a <code>details</code> block is open, it has the <code>open</code> attribute that I mentioned earlier. To style it (or its children) based on its state, use <code>details[open] { }</code>.</p>
<p>Note: there's no <code>closed</code> attribute: styles you apply by "default" will be used on the closed state.</p>
</details>

<details>
  <summary>But this requires JavaScript, right?</summary>
  <p>Open the JS panel on this pen. Clean as a whistle! This is handled totally by the browser.</p>
</details>

<details>
  <summary>What about accessibility? Is that the catch?</summary>
  <p>Sorry to disappoint you. Since these are native HTML elements, they provide useful semantic information to screen readers.</p>
  <p>Screen readers will typically read the <code>summary</code> for a collapsed <code>details</code> block (and communicate that it's collapsed). They'll also provide an interactive hook for users to open the <code>details</code> block. If the <code>details</code> is already expanded, they'll read the whole content.</p>
  <p>I don't rely on assistive tech to read the web, so I'm probably not aware of some limitations or drawbacks to using <code>details</code> and <code>summary</code>, but I suspect their AX is at least as good as (if not better than) most JavaScript-dependent accordion solutions.</p>
</details>

<details>
  <summary>Excellent! More information, please!</summary>
  <p>You bet! Here are some great resources on <code>details</code> &amp; <code>summary</code>:</p>
  <ul>
    <li>MDN: <a href="https://developer.mozilla.org/en-US/docs/Web/HTML/Element/details" target="_blank" rel="noopener">details</a>, <a href="https://developer.mozilla.org/en-US/docs/Web/HTML/Element/summary" target="_blank" rel="noopener">summary</a></li>
    <li><a href="http://html5 doctor.com/the-details-and-summary-elements/" target="_blank" rel="noopener">HTML5Doctor</a></li>
    <li><a href="https://www.scottohara.me/blog/2018/09/03/details-and-summary.html" target="_blank" rel="noopener">Scott O'Hara</a></li>
    <li><a href="https://blog.teamtreehouse.com/use-details-summary-elements" target="_blank" rel="noopener">Treehouse Blog</a></li>
    <li><a href="https://webdesign.tutsplus.com/tutorials/explaining-the-details-and-summary-elements--cms-21999" target="_blank" rel="noopener">Envato Tuts+</a></li>
    <li><a href="https://caniuse.com/#feat=details" target="_blank" rel="noopener">caniuse table for <code>details</code></a></li>
  </ul>
</details>

<details>
  <summary>But what about cross-browser compatibility?</summary>
  <p>Yeah, sorry. Here's some bad news. IE, Edge, and Opera Mini don't currently support <code>details</code>/<code>summary</code> with native open/close behavior (check out <a href="https://caniuse.com/#feat=details" target="_blank" rel="noopener">caniuse data for <code>details</code></a>).<p>
  <p>These browsers will show all your <code>details</code> elements expanded. That's not the worst though: it's a bit of progressive enhancement: if the browser doesn't support the native UI behavior, the content will still be visible to users.</p>
  <p>It's unlikey IE11 will be getting any updates on this front, but there's hope for Edge! If this is important to you, please <a href="https://wpdev.uservoice.com/forums/257854-microsoft-edge-developer/suggestions/6261266-details-summary-elements" target="_blank" rel="noopener">cast a vote a vote for Edge to support <code>details</code>/<code>summary</code></a>. (Or just wait for Edge to use Chromium, I guess. 😕)</p>
  <p>If you do need to have open/close behavior in IE11 (or any other non-supporting browser), you'll probably need a polyfill. This <a href="https://www.smashingmagazine.com/2014/11/complete-polyfill-html5-details-element/" target="_blank" rel="noopener">Smashing Magazine <code>details</code> polyfill tutorial</a> looks like a good place to start.</p>
</details>
!

CSS 

              
                *,
::before,
::after {
  box-sizing: border-box;
}

html {
  background-color: #CFD8DC;
}

details {
  margin: 1rem auto;
  padding: 0 1rem;
  width: 35em;
  max-width: calc(100% - 2rem);
  position: relative;
  border: 1px solid #78909C;
  border-radius: 6px;
  background-color: #ECEFF1;
  color: #263238;
  transition: background-color .15s;
  
  > :last-child {
    margin-bottom: 1rem;
  }
  
  &::before {
    width: 100%;
    height: 100%;
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    border-radius: inherit;
    opacity: .15;
    box-shadow: 0 .25em .5em #263238;
    pointer-events: none;
    transition: opacity .2s;
    z-index: -1;
  }
  
  &[open] {
    background-color: #FFF;
    
    &::before {
      opacity: .6;
    }
  }
}

summary {
  padding: 1rem 2em 1rem 0;
  display: block;
  position: relative;
  font-size: 1.33em;
  font-weight: bold;
  cursor: pointer;
  
  &::before,
  &::after {
    width: .75em;
    height: 2px;
    position: absolute;
    top: 50%;
    right: 0;
    content: '';
    background-color: currentColor;
    text-align: right;
    transform: translateY(-50%);
    transition: transform .2s ease-in-out;
  }
  
  &::after {
    transform: translateY(-50%) rotate(90deg);
    
    [open] & {
      transform: translateY(-50%) rotate(180deg);
    }
  }
  
  &::-webkit-details-marker {
    display: none;
  }
}

p {
  margin: 0 0 1em;
  line-height: 1.5;
}

ul {
  margin: 0 0 1em;
  padding: 0 0 0 1em;
}

li:not(:last-child) {
  margin-bottom: 0.5em;
}

code {
  padding: 0.2em;
  border-radius: 3px;
  background-color: #E0E0E0;
  
  pre > & {
    display: block;
    padding: 1em;
    margin: 0;
  }
}
!

JS 

              
                // Nothing to see here…
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999px

Console