body {
    font-family:Arial, Helvetica, sans-serif;
}
.CodeMirror, #editor, #preview {
    width:50%;
    float:left;
    height:calc(100vh - 50px);
}
.CodeMirror, #preview {
    box-sizing: border-box;
    border:1px solid #424242;
}
header {
    height:50px;
    line-height: 50px;
}
.col {
    flex: 50%;
}
.row {
    display: flex;
    justify-content: space-between;
}
.status {
    min-width: 50px;
    text-align: center;
}
.save {
    margin-right:10px;
}
.status, .save {
    height:50px;
    padding:0 10px;
    display: inline-block;
    float:right;
}
button.save {
    border:0;
    background: #0074D9;
    color: hsla(208, 100%, 85%, 1.0);
}
button.save:hover {
    color:#000;
    background:#7FDBFF;
    transition: background 1s;
    cursor: pointer;
}
button.save:disabled {
    background: #ddd;
}
.pass {
    background:green;
    color:#fff;
}
.fail {
    background:red;
    color:#fff;
}

body {
    font-family:Arial, Helvetica, sans-serif;
}
.CodeMirror, #editor, #preview {
    width:50%;
    float:left;
    height:calc(100vh - 50px);
}
.CodeMirror, #preview {
    box-sizing: border-box;
}
#preview {
    border:none;
}
header {
    height:50px;
    line-height: 50px;
    box-shadow: inset 0 -1px 0 0 rgba(100,121,143,0.122);
}
.col {
    flex: 50%;
}
.row {
    display: flex;
    justify-content: space-between;
}
.status {
    min-width: 50px;
    text-align: center;
}
.save {
    margin-right:10px;
}
.status, .save {
    height:50px;
    padding:0 10px;
    display: inline-block;
    float:right;
}
button.save {
    border:0;
    background: #0074D9;
    color: hsla(208, 100%, 85%, 1.0);
}
button.save:hover {
    color:#000;
    background:#7FDBFF;
    transition: background 1s;
    cursor: pointer;
}
button.save:disabled {
    background: #ddd;
}
.pass {
    background:green;
    color:#fff;
}
.fail {
    background:red;
    color:#fff;
}

Remove borders and add a box shadow
Remove borders and add a box shadow
