editor.css

body {
    margin: 0;
    padding: 0;
    overflow: hidden;
}
header {
    height: 32px;
    line-height: 32px;
    background: gray;
    padding: 0 10px;
    box-sizing: border-box;
}
#editor {
    width: 50vw;
    height: calc(100vh - 32px);
    box-sizing: border-box;
    float: left;
    border-right: 1px solid white;
}
#output {
    color: white;
    background: black;
    width: 50vw;
    height: calc(100vh - 32px);
    box-sizing: border-box;
    overflow: auto;
    float: right;
    border-left: 1px solid white;
    white-space: pre;
}