The HTML/CSS notepad is a tool you can use to test your “HTML code”. While you can certainly do this in your browser by creating a new file and opening it in your browser, this is simply a quicker way to do it. By default, you have the standard HTML skeleton and a <style> tag where you can place your CSS. This tool is only intended for quick testing of HTML and CSS. If you are actually designing a website, please use your browser as a testing tool.
Your code will execute in this window.