* {box-sizing: border-box}
body {margin: 0;}
#editor-textarea {width: 100%;resize: none;padding: 10px;height: 450px;}
    #editor-textarea:focus {outline: none}
#viewer {background-color: #fff;width: 100%;}

@media only screen and (max-width : 600px) {
    #viewer,#editor-textarea {width: 100%;height: 50%;}
    .main {display: block;width: 100%}
}