Paste the following code into your browser address bar:
data:text/html,<body ><style>textarea,iframe{width:100%;height:50%}body{margin:0}textarea{width:33.33%;font-size:18}</style><textarea placeholder=HTML id=h></textarea><textarea placeholder=CSS id=c></textarea><textarea placeholder=JS id=j></textarea><iframe id=i>
If you want to read the code, check out index.html