What is Liveness in JavaScript?

Question

Trying to examine intricacies of JavaScript GC, I got deep into the weeds (that is, into the ECMAScript spec). It was found by me that an object should not be collected as long as it is deemed "live". And liveness itself is defined as follows:

At any point during evaluation, a set of objects S is considered live if either of the following conditions is met:

• Any element in S is included in any agent's [[KeptAlive]] List.
• There exists a valid future hypothetical WeakRef-oblivious execution with respect to S that observes the Object value of any object in S.

The [[KeptAlive]] list is appended with an object once a special WeakRef is created, which (weakly) refers to it, and emptied after the current synchronous job ceases. However, as for WeakRef-oblivious execution, I fail to get my mind around what it is:

For some set of objects S, a hypothetical WeakRef-oblivious execution with respect to S is an execution whereby the abstract operation WeakRefDeref of a WeakRef whose referent is an element of S always returns undefined.

WeakRefDeref of a WeakRef returns undefined when its referent was collected already. Am I getting it right that it is implied here that all objects that make up S should be collected? So the notion of a future hypothetical WeakRef-oblivious execution is that there is still an object, an element of S, which not collected yet and observed by some WeakRef.

It all still makes little sense for me. I would appreciate some samples.