:root {
  --bg1: #fd5;
  --bg2: #fd5;
  --fg1: #000;
  --fg2: #fff;
}

* {
  box-sizing: border-box;
}

body {
  height: 100vh;
  display: flex;
  justify-content: center;
  align-items: center;
  font: normal 19px/1.5 helvetica, sans-serif;
}

.container {
  width: 500px;
  height: 400px;
  position: relative;
}

.top {
  position: absolute;
  top: -48px;
  font-size: .7em;
}
.top h3 {
  margin: 0;
  padding: 0;
  margin-bottom: .5em;
  color: #333;
  display: inline-block;
}
.top label {
  margin-left: 1.75em;
}
.top input {
  margin-left: .1em;
}

.bottom {
  position: relative;
  width: 500px;
  height: 400px;
  -webkit-user-select: none;
     -moz-user-select: none;
      -ms-user-select: none;
          user-select: none;
}
.bottom .left, .bottom .right {
  position: absolute;
  top: 0;
  left: 0;
  height: 400px;
  overflow: hidden;
}
.bottom .left .content, .bottom .right .content {
  width: 500px;
  padding: 3em;
}
.bottom .left {
  width: 50%;
  max-width: 100%;
  min-width: 10px;
  resize: horizontal;
  z-index: 1;
  -webkit-animation: demo 4.5s 1;
          animation: demo 4.5s 1;
}
.bottom .left .resizable-handle {
  width: 8px;
  height: 100%;
  background: rgba(255, 238, 204, 0.5);
  border-left: 1px solid rgba(255, 255, 255, 0.75);
  border-right: 1px solid rgba(0, 0, 0, 0.06125);
  border-radius: 5px;
  box-shadow: 0 0 5px rgba(0, 0, 0, 0.06125);
  transition: all .2s;
}
.bottom .left .resizable-handle:hover {
  width: 10px;
  box-shadow: 0 0 10px rgba(0, 0, 0, 0.125);
}
.bottom .left .resizable-handle-active {
  width: 13px !important;
  background: rgba(255, 238, 204, 0.65);
}
.bottom .left {
  background: var(--bg1);
  color: var(--fg1);
}
.bottom .right {
  background: var(--bg2);
  color: var(--fg2);
}
.bottom h1 {
  margin-top: 0;
}
.bottom .small {
  font-size: .75em;
}

@-webkit-keyframes demo {
  12.5% {
    width: 80%;
  }
  50% {
    width: 25%;
  }
  80% {
    width: 50%;
  }
}

@keyframes demo {
  12.5% {
    width: 80%;
  }
  50% {
    width: 25%;
  }
  80% {
    width: 50%;
  }
}