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HTML

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Adding Classes

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About the <head>

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Insecure Resource

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CSS

About CSS Preprocessors

CSS preprocessors help make authoring CSS easier. All of them offer things like variables and mixins to provide convenient abstractions.

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CSS Base

About CSS Base

It's a common practice to apply CSS to a page that styles elements such that they are consistent across all browsers. We offer two of the most popular choices: normalize.css and a reset. Or, choose Neither and nothing will be applied.

Vendor Prefixing

About Vendor Prefixing

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JavaScript

About JavaScript Preprocessors

JavaScript preprocessors can help make authoring JavaScript easier and more convenient.

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Babel includes JSX processing.

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HTML 

              
                <div class="container">
  <h2>Frequently Asked Questions</h2>
  <div class="accordion">
    <div class="accordion-item">
      <button id="accordion-button-1" aria-expanded="false"><span class="accordion-title">Why is the moon sometimes out during the day?</span><span class="icon" aria-hidden="true"></span></button>
      <div class="accordion-content">
        <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Elementum sagittis vitae et leo duis ut. Ut tortor pretium viverra suspendisse potenti.</p>
      </div>
    </div>
    <div class="accordion-item">
      <button id="accordion-button-2" aria-expanded="false"><span class="accordion-title">Why is the sky blue?</span><span class="icon" aria-hidden="true"></span></button>
      <div class="accordion-content">
        <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Elementum sagittis vitae et leo duis ut. Ut tortor pretium viverra suspendisse potenti.</p>
      </div>
    </div>
    <div class="accordion-item">
      <button id="accordion-button-3" aria-expanded="false"><span class="accordion-title">Will we ever discover aliens?</span><span class="icon" aria-hidden="true"></span></button>
      <div class="accordion-content">
        <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Elementum sagittis vitae et leo duis ut. Ut tortor pretium viverra suspendisse potenti.</p>
      </div>
    </div>
    <div class="accordion-item">
      <button id="accordion-button-4" aria-expanded="false"><span class="accordion-title">How much does the Earth weigh?</span><span class="icon" aria-hidden="true"></span></button>
      <div class="accordion-content">
        <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Elementum sagittis vitae et leo duis ut. Ut tortor pretium viverra suspendisse potenti.</p>
      </div>
    </div>
    <div class="accordion-item">
      <button id="accordion-button-5" aria-expanded="false"><span class="accordion-title">How do airplanes stay up?</span><span class="icon" aria-hidden="true"></span></button>
      <div class="accordion-content">
        <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Elementum sagittis vitae et leo duis ut. Ut tortor pretium viverra suspendisse potenti.</p>
      </div>
    </div>
  </div>
</div>
!

CSS 

              
                @import url('https://fonts.googleapis.com/css?family=Hind:300,400&display=swap');

$bg: #fff;
$text: #7288a2;
$gray: #4d5974;
$lightgray: #e5e5e5;
$blue: #03b5d2;

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

body {
  margin: 0;
  padding: 0;
  font-family: 'Hind', sans-serif;
  background: $bg;
  color: $gray;
  display: flex;
  min-height: 100vh;
}

.container {
  margin: 0 auto;
  padding: 4rem;
  width: 48rem;
}

.accordion {
  .accordion-item {
    border-bottom: 1px solid $lightgray;
    button[aria-expanded='true'] {
      border-bottom: 1px solid $blue;
    }
  }
  button {
    position: relative;
    display: block;
    text-align: left;
    width: 100%;
    padding: 1em 0;
    color: $text;
    font-size: 1.15rem;
    font-weight: 400;
    border: none;
    background: none;
    outline: none;
    &:hover, &:focus {
      cursor: pointer;
      color: $blue;
      &::after {
        cursor: pointer;
        color: $blue;
        border: 1px solid $blue;
      }
    }
    .accordion-title {
      padding: 1em 1.5em 1em 0;
    }
    .icon {
      display: inline-block;
      position: absolute;
      top: 18px;
      right: 0;
      width: 22px;
      height: 22px;
      border: 1px solid;
      border-radius: 22px;
      &::before {
        display: block;
        position: absolute;
        content: '';
        top: 9px;
        left: 5px;
        width: 10px;
        height: 2px;
        background: currentColor;
      }
      &::after {
        display: block;
        position: absolute;
        content: '';
        top: 5px;
        left: 9px;
        width: 2px;
        height: 10px;
        background: currentColor;
      }
    }
  }
  button[aria-expanded='true'] {
    color: $blue;
    .icon {
      &::after {
        width: 0;
      }
    }
    + .accordion-content {
      opacity: 1;
      max-height: 9em;
      transition: all 200ms linear;
      will-change: opacity, max-height;
    }
  }
  .accordion-content {
    opacity: 0;
    max-height: 0;
    overflow: hidden;
    transition: opacity 200ms linear, max-height 200ms linear;
    will-change: opacity, max-height;
    p {
      font-size: 1rem;
      font-weight: 300;
      margin: 2em 0;
    }
  }
}
!

JS 

              
                const items = document.querySelectorAll(".accordion button");

function toggleAccordion() {
  const itemToggle = this.getAttribute('aria-expanded');
  
  for (i = 0; i < items.length; i++) {
    items[i].setAttribute('aria-expanded', 'false');
  }
  
  if (itemToggle == 'false') {
    this.setAttribute('aria-expanded', 'true');
  }
}

items.forEach(item => item.addEventListener('click', toggleAccordion));
!
999px

Console