WebIDL is not a replacement for the DOM. Instead, it is a language used for specifying APIs, including the most recent version of the DOM API as well as other recent web standards.
The DOM4 standard contains WebIDL code for all of the interfaces and types it defines. For example, take a look at the WebIDL definition of the DOM ParentNode interface. It's just a format for listing the properties and methods of this interface. The IDL lists the name of each method, its return type and parameters, and a few other pieces of information about it. When you write document.body.childElementCount
you are accessing a DOM API, and you can read the WebIDL code in the DOM standard to find out what type of data that API will return.
If you are a web developer, WebIDL doesn't change how you write your code at all, and it doesn't necessarily change anything about how browsers run your code. It's just a way for browser developers to specify interfaces in a standard, language-independent way.
"Appendices" (plural of "appendix") are sections or chapters of a document that are appended after the main content. A "JS language binding appendix" is an addition to an IDL spec that defines how the IDL interfaces should look when accessed by JavaScript programs.