styles.css

html {
  box-sizing: border-box;
  width: 100%;
  height: 100%;
}
*, *:before, *:after {
  box-sizing: inherit;
}
body {
  margin: 0;
  padding: 0;
  width: 100%;
  height: 100%;
  font-family: Arial, Verdana;
  color: #000;
  background: #fff;
  font-size: 16px;
}
.container {
  width: 100%;
  height: 100%;
  position: relative;
}
.left, .right {
  width: 50%;
  height: 100%;
}
.left {
  float: left;
  border-right: 1px solid #999;
}
.right {
  float: right;
  padding: 0.5em;
}
.CodeMirror {
  height: auto;
  transition: opacity 300ms;
  -webkit-transition: opacity 300ms;
  opacity: 1;
}
.half {
  height: 60%;
  border-bottom: 1px solid #FFF;
  transition: border-bottom 200ms ease;
  -webkit-transition: border-bottom 200ms ease;
}
.half:nth-child(2) {
  height: 40%;
  border-top: 1px solid #999;
  font-size: 0.8em;
}
[data-status="error"] {
  border-bottom: 42px solid #F2D8D8;
}
.error {
  display: none;
  position: absolute;
  top: calc(60% - 40px);
  left: 0;
  z-index: 100;
  color: #333;
  font-size: 0.8em;
  width: 50%;
  padding: 0.4em;
}