It seems doing:

callback interface X {
  handleEvent(...);
}

[Custom]
callback interface X {
  handleEvent(...);
}

callback interface X {
  [Custom] handleEvent(...);
}

callback Foo = void ();

[Custom] callback Foo = void ();

All generate subtlety different code, and almost all users are actually creating custom bindings of them. The non custom version of callbacks appears to actually be broken (See  issue 658086 ).

There's a bunch of callbacks now, and also observer interfaces

IntersectionObserver => [Custom] callback, CustomConstructor on observer.
ResizeObserver => regular callback interface with [Custom] void handleEvent, no custom observer.
PerformanceObserver => nothing custom (this is broken today).
MutationObserver => CustomConstructor(MutationCallback), confusingly MutationCallback is not defined in idl at all, and is totally in C++ as custom code. It seems the idl compiler didn't type check the inputs, it probably should and yell when you pass something that's undefined.
IDBObserver => callback, CustomConstructor (this is broken today).