<!doctype html>
<html>
<head>
<meta charset="UTF-8">
<title>My Web Page</title>
<script>
  alert('hello world!');
</script>
</head>

Note:

On the upper right side corner three buttons are given:

This image has an empty alt attribute; its file name is buttons.png

The first button is used to run and execute the code.

Second Button is used for full screen mode.

And the third button is used to copy the Javascript code.