There have always been undocumented features in the web browsers. Some of these were uncovered during the preparation of this book and have been covered in their own topics.
Generally, the undocumented features are methods and properties of objects that are not commonly used and maybe an extension to the object that is platform specific. There are also objects that are obscure, and can only be discovered by inspection.
Reading the standards documents, release notes, and manufacturer manuals, only reveals part of the picture.
Armed with this knowledge, you can write inspection scripts to display the values of those properties. Often they point at an object and if the property is undocumented, then it's likely the object is as well.