Flex panels in CSS and JS

I was following Wes Bos JS 30-day challenge, so HTML and CSS are mostly are copy-paste, I'd like feedback on JS (mostly). Thanks.

const panels = document.querySelectorAll('.panel');

panels.forEach(panel => panel.addEventListener('click', () => {
  const isOpen = panel.classList.contains('open');
  panels.forEach(panel => panel.classList.remove('open'));
  if(!isOpen) {
    panel.classList.add('open');
  }
}));

panels.forEach(panel => panel.addEventListener('transitionend', e => {
  if(e.propertyName.includes('flex')) {
    panels.forEach(panel => {
      if(panel.classList.contains('open')) {
        panel.classList.add('open-active');
      } else {
        panel.classList.remove('open-active');
      }
    });
  }
}));

html {
  box-sizing: border-box;
  background: #ffc600;
  font-family: 'helvetica neue';
  font-size: 20px;
  font-weight: 200;
}

body {
  margin: 0;
}

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

.panels {
  min-height: 100vh;
  overflow: hidden;
  display: flex;
}

.panel {
  background: #6B0F9C;
  box-shadow: inset 0 0 0 5px rgba(255, 255, 255, 0.1);
  color: white;
  text-align: center;
  align-items: center;
  /* Safari transitionend event.propertyName === flex */
  /* Chrome + FF transitionend event.propertyName === flex-grow */
  transition:
    font-size 0.7s cubic-bezier(0.61, -0.19, 0.7, -0.11),
    flex 0.7s cubic-bezier(0.61, -0.19, 0.7, -0.11), background 0.2s;
  font-size: 20px;
  background-size: cover;
  background-position: center;

  flex: 1;
  display: flex;
  justify-content: center;
  flex-direction: column;
}

.panel1 {
  background-image: url(https://source.unsplash.com/gYl-UtwNg_I/1500x1500);
}

.panel2 {
  background-image: url(https://source.unsplash.com/rFKUFzjPYiQ/1500x1500);
}

.panel3 {
  background-image: url(https://images.unsplash.com/photo-1465188162913-8fb5709d6d57?ixlib=rb-0.3.5&q=80&fm=jpg&crop=faces&cs=tinysrgb&w=1500&h=1500&fit=crop&s=967e8a713a4e395260793fc8c802901d);
}

.panel4 {
  background-image: url(https://source.unsplash.com/ITjiVXcwVng/1500x1500);
}

.panel5 {
  background-image: url(https://source.unsplash.com/3MNzGlQM7qs/1500x1500);
}

/* Flex Children */
.panel>* {
  margin: 0;
  width: 100%;
  transition: transform 0.5s;
  flex: 1 0 auto;
  display: flex;
  align-items: center;
  justify-content: center;
}

.panel *:first-child {
  transform: translateY(-100%);
}

.panel *:last-child {
  transform: translateY(100%);
}

.panel.open-active *:first-child,
.panel.open-active *:last-child {
  transform: translateY(0);
}

.panel p {
  text-transform: uppercase;
  font-family: 'Amatic SC', cursive;
  text-shadow: 0 0 4px rgba(0, 0, 0, 0.72), 0 0 14px rgba(0, 0, 0, 0.45);
  font-size: 2em;
}

.panel p:nth-child(2) {
  font-size: 4em;
}

.panel.open {
  font-size: 40px;
  flex: 5;
}

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Flex Panels 💪</title>
  <link href="https://fonts.googleapis.com/css?family=Amatic+SC" rel="stylesheet" type="text/css">
</head>
<body>

  <div class="panels">
    <div class="panel panel1">
      <p>Hey</p>
      <p>Let's</p>
      <p>Dance</p>
    </div>
    <div class="panel panel2">
      <p>Give</p>
      <p>Take</p>
      <p>Receive</p>
    </div>
    <div class="panel panel3">
      <p>Experience</p>
      <p>It</p>
      <p>Today</p>
    </div>
    <div class="panel panel4">
      <p>Give</p>
      <p>All</p>
      <p>You can</p>
    </div>
    <div class="panel panel5">
      <p>Life</p>
      <p>In</p>
      <p>Motion</p>
    </div>
  </div>

</body>
</html>

javascript css

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Flex panels in CSS and JS