<div class="play-pause"></div>

html, body {
  height: 100%;
  margin: 0;
  background: #222;
}

body {
  font-size: 100px;
  display: flex;
  align-items: center;
  justify-content: center;
}

.play-pause {
  /* This must match the background color behind the button */
  color: #222;
  /* This is the color of the button */
  background-color: #f06;

  width: 1em; height: 1em;
  position: relative;
  cursor: pointer;
  overflow: hidden;

  /* Draw the 2 pause bars */
  background-image: linear-gradient(to right,
                                    transparent 40%,
                                    currentColor 40%,
                                    currentColor 60%,
                                    transparent 0);
  background-position: center;
  background-repeat: no-repeat;
  background-size: 100% 100%;

  /* Hide the central part between the 2 bars */
  transition: background-size .3s;
}

.play-pause.paused {
  background-size: 0 100%;
}

.play-pause::before,
.play-pause::after {
  content: "";
  position: absolute;
  top: 0;
  height: 100%;
  width: 100%;
  background: currentColor;
  /* Before and after are skewed at 90deg to be invisible */
  transform: skew(90deg);
  transform-origin: top left;

  /* And transitioned to the right angle to transform the square into a
     triangle */
  transition: transform .3s;
}

.play-pause::after {
  transform-origin: bottom left;
}

.play-pause.paused::before {
  transform: skew(64deg);
}

.play-pause.paused::after {
  transform: skew(117deg);
}

document.querySelector(".play-pause").addEventListener("click", function() {
  this.classList.toggle("paused");
});