style.css

body {
    margin: 0px;
    font-family: "Raleway",sans-serif;
}
.header {
    width: 100%;
    height: 50px;
    background: #111;
    display: flex;
    justify-content: center;
    align-items: center;
}
.header button{
    padding: 10px 20px;
    background: #f5f5f5;
    color: #111;
    font-size: 16px;
    font-weight: 600;
    border: none;
    outline: none;
    border-radius: 0px;
    cursor: pointer;
}
.editor {
    width: 100%;
    height: calc(100vh - 50px);
    display: flex;
}
.editor .code{
    width: 500px;
    border-right: 1px solid #888;
}
.editor .code > div {
    width: 100%;
    height: 33.33%;
    border-bottom: 1px solid #888;
}
.editor .preview{
    width: calc(100% - 500px);
    height: 100%;
}
.editor .preview iframe{
    width: 100%;
    height: 100%;
    border: none;
    outline: none;
}
.CodeMirror {
    height: auto;
}