style.css

@import url("https://fonts.googleapis.com/css2?family=Roboto:wght@400;700&display=swap");

* {
  box-sizing: border-box;
}

body {
  background-color: #efedd696;
  font-family: "Roboto", sans-serif;
  display: flex;
  /* flex here aligns the content to the centre of the window */
  flex-direction: column;
  align-items: center;
  justify-content: center;
  margin: 0;
  overflow-x: hidden;
}

h1 {
  margin: 10px;
}

.box {
  background-color: steelblue;
  color: #fff;
  display: flex;
  align-items: center;
  justify-content: center;
  width: 400px;
  height: 200px;
  margin: 10px;
  border-radius: 10px;
  box-shadow: 2px 4px 5px rgba(0, 0, 0, 0.3);
  /* the transform moves the content box off screen
    400% is large enough for even large screen sizes */
  transform: translateX(400%);
  /* changed ease from 0.4 to 0.8 as looked a bit too quick */
  transition: transform 0.8s ease;
}

.box:nth-of-type(even) {
  /* this moves every other content box to the opposite side */
  transform: translateX(-400%);
}

.box.show {
  /* the .show class moves the content box to default position - 0 */
  transform: translateX(0);
}

.box h2 {
  font-size: 45px;
}