Paste the following code into your browser address bar:
data:text/html,<body oninput="i.srcdoc=h.value+'<style>'+c.value+'</style><script>'+j.value+'<\/script>'"><style>[id]{width:100%;height:50%}*{margin:0}:valid{width:33.33%;font-size:18}<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]
And remember, every byte counts.