<ul>
  <li>
    <div class="section-title">
      <h2>Section 1</h2>
    </div>
    <div class="section-content">
      <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Suscipit voluptatum temporibus dicta reprehenderit tempore quisquam consequuntur porro omnis laboriosam praesentium at et sapiente, provident sit! Suscipit recusandae, ab ratione dignissimos.</p>
    </div>
  </li>
  <li class="active">
    <div class="section-title">
      <h2>Section 2</h2>
    </div>
    <div class="section-content">
      <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Tempora totam delectus, eius nostrum aspernatur voluptas enim fugit ipsa magni voluptatem, odio sit accusamus vel id, commodi consequuntur possimus repellat necessitatibus!</p>
    </div>
  </li>
  <li>
    <div class="section-title">
      <h2>Section 3</h2>
    </div>
    <div class="section-content">
      <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Pariatur saepe vel facilis quae nihil ad aspernatur ex delectus. Tenetur nulla voluptates similique quos, quia possimus, magnam esse natus quis ipsa.</p>
    </div>
  </li>
  <li>
    <div class="section-title">
      <h2>Section 4</h2>
    </div>
    <div class="section-content">
      <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Adipisci, optio illo, delectus deleniti animi accusamus. Laboriosam maiores totam provident aliquam. Unde, incidunt amet officia a obcaecati, ducimus at molestiae nemo.</p>
    </div>    
  </li>
</ul>
* {
  box-sizing: border-box;
/*   border: 1px solid black; */
}

body {
  font-family: 'Lato';
   -webkit-font-smoothing: antialiased;
	-moz-font-smoothing: antialiased;
	-moz-osx-font-smoothing: grayscale;
	text-rendering: optimizelegibility;
}

ul {
  display: flex;
  min-height: 100vh;
  max-height: 100vh;
  margin: 0;
  padding: 0;
  overflow: hidden;
  list-style-type: none;
}

li {
  flex: 1;
  display: flex;
  align-items: stretch;
  padding: 20px;
  background: #E74C3C;
  cursor: pointer;
  transition: all .5s ease;
  
  &:hover {
    background: darken(#E74C3C, 3);
  }
  
  &.active {
    flex: 5;
    background: #fff;
    cursor: default;
    
    h2 {
      color: #E74C3C;
    }
    
    .section-content {
      flex: 4;
      opacity: 1;
      transform: scaleX(1);
      color: #E74C3C;
    }
  }
  
  .section-title {
    flex: 1;
    display: flex;
    align-items: center;
    width: 100%;
    margin: 0;
    padding: 0;
    text-align: center;
    color: white;
    
    h2 {
      margin: 0;
      transform: rotate(-90deg);
      white-space: nowrap;
    }
  }
  
  .section-content {
    flex: 1;
    display: flex;
    align-items: center;
    margin: 0;
    padding: 0;
    opacity: 0;
    transition: all .25s .1s ease-out;
  }
}

View Compiled
var section = $('li');

function toggleAccordion() {
  section.removeClass('active');
  $(this).addClass('active');
}

section.on('click', toggleAccordion);

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

  1. https://cdnjs.cloudflare.com/ajax/libs/jquery/2.1.3/jquery.min.js