It's a little hard to tell what exactly you are trying to achieve (the screenshot is rather small), but it could be related to this.
It's done using the preon-emitter
module, emitting meta data on the different snippets of data it decoded, while the decoder is running. As you can see, it allows you to build an HTML page that - while you're hovering over the bytes - gives you the information about the data structure it decoded.
In this particular case, it's showing the contents of a class file. The bold word at the left is the value decoded, the rest is where that value resides in the class files data structure.