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 +
        "" +
        ""
    );
    code.close();
  };
}

compile();