That is not what most people would consider a text editor, it is part of the learning tools at w3schools.com. They let you make changes to the lesson at hand and view the results. On their site. You can do the same thing on your desktop with something like notepad++ or PSPad or Arachnophilia or any of dozens of
HTML editors and you can edit real pages instead of lessons - and view the changes instantly.
It is a great feature that makes learning
html fairly easy on their site, it lets you practice what they preach.