script.js

function compile() {
 var html = document.getElementById('html');
 var css = document.getElementById('css');
 var js = document.getElementById('js');
 var code = document.getElementById('code').contentWindow.document;

 document.body.onkeyup = function () {
 code.open();
 code.writeln(
 html.value +
 '<style>' +
 css.value +
 '</style>' +
 '<script>' +
 js.value +
 '</script>'
 );
 code.close();
 };
}

compile();