<p>Can you guess how the animation is working without reading the source code?</p>

<input type="checkbox" id="toggle_checkbox">
<label for="toggle_checkbox"></label>

* {
  -webkit-tap-highlight-color: transparent;
}

html,
body {
  height: 100%;
}

body {
  margin: 0;
  background-color: #f3fbff;
}

p {
  position: fixed;
  top: 0;
  right: 0;
  left: 0;
  padding: 10px;
  color: #143240;
  text-align: center;
  font-size: 14px;
  font-family: Helvetica, serif;
}

#toggle_checkbox {
  display: none;
}

label {
  position: absolute;
  top: 50%;
  right: 0;
  left: 0;
  display: block;
  width: 140px;
  height: 70px;
  margin: 0 auto;
  transform: translateY(-50%);
  cursor: pointer;
}

label:before {
  top: 8px;
  left: 8px;
  width: 54px;
  height: 54px;
  border: 3px solid red;
}

label:after {
  top: 0;
  right: 0;
  width: 140px;
  height: 70px;
  border: 3px solid #143240;
}

label:before,
label:after {
  content: "";
  position: absolute;
  border-radius: 50px;
  box-sizing: border-box;
  transition: 0.5s ease top, 0.5s ease left, 0.5s ease right, 0.5s ease width,
    0.5s ease height, 0.5s ease border-color;
}

#toggle_checkbox:checked + label:before {
  top: 0;
  left: 0;
  width: 140px;
  height: 70px;
  border-color: #143240;
}

#toggle_checkbox:checked + label:after {
  top: 8px;
  right: 8px;
  width: 54px;
  height: 54px;
  border-color: red;
}