A live text editor to do a quick test for HTML5/CSS3/JavaScript and also a HTML5 based presentation maker
You need a webserver to run this such as XAMPP/LAMPP/MAMP because the index was written in PHP. Copy and paste all the files to a folder in your htdocs or www folder and run it from your localhost/yourfoldername
If you using windows you can build a standalone app by using this resource (https://github.com/cztomczak/phpdesktop)
For Linux and Mac you can use Electron - (https://github.com/electron/electron)