Blackbird is a utility which lets you write and test little bits of HTML code. Type (or paste) your HTML code and it will render the preview.
It's great for testing code, or for learning the basics of how HTML works.
Blackbird does not collect any identifying information. If you have enabled crash reporting for all apps on your Mac, the developer will receive anonymous crash reports to help improve the stability of the app. No specific information (such as html entered) are read or stored off of the device.