styles.css

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

body {
  height: 100vh;
}

.portfolio {
  width: 100%;
  height: 100%;
  position: relative;
  overflow: hidden;
  transition: all .7s;
  margin: auto;
}

.layer-group {
  width: 100%;
  height: 100%;
  position: absolute;
  left: 0;
  display: flex;
  flex-direction: column;
  flex-wrap: wrap;
  transition: all .7s;
}

.layer {
  width: 100%;
  height: 100%;
  font-size: 0;
  display: none;
  position: absolute;
  transition: all .7s;
  background: purple;
}

.layer:first-child {
  display: block;
}

iframe {
  width: 33.33%;
  height: 50%;
  border: none;
}

.control {
  position: absolute;
  width: 80px;
  height: 100%;
  z-index: 10;
  display: flex;
  justify-content: center;
  align-items: center;
  flex-direction: column;
  background-color: rgba(255, 255, 255, 0.2);
}

.control.next { right: 0; }
.control .arrow { position: relative; }
.control.prev .arrow { left: 7px; }

.control.next .arrow {
  transform: rotate(180deg);
  transform-origin: top;
  right: 7px;
}

.control .arrow:after,
.control .arrow:before {
  content: '';
  border-width: 2px 0 0 0;
  border-style: solid;
  border-color: #fff;
  width: 60px;
  display: block;
  transform: rotate(45deg);
  transform-origin: left;
}

.control .arrow:after { transform: rotate(-45deg); }

.control:hover { background-color: rgba(255, 255, 255, 0.3); }

.control:active {
  background-color: rgba(255, 255, 255, 0.4);
  transform: scale(1.1);
}