let id = response.headers.get("X-Request-ID");
if (id) {
  let info = this._pendingRequests.get(id);
  if (info) {

This is unused.

Unused. Caller can/should be passing this in so we can handle the quota check internally.

The implicitness that this "protocol" pushes into the implementing methods (seemingly odd return values) is undesirable. Make it explicit. The implementing methods should be able to do something like Validator.changeExpectation(this, this.onresource) (which in turn changes the target's ontokencontent handler).

Unnecessary check.

Better to use querySelectorAll and then filter based on figure designation (e.g. nodeValue.startsWith(" (Fig. 1) ")).

Unused!

(Caller should also probably look more like:

let system = new SystemB(document);
system.attach(document.querySelectorAll("script"))

... where the querySelectorAll return value's role is to act as a list of document figures, for the reasons described elsewhere https://hypothes.is/a/-n-RYt4WEeu5WIejr9cfKA.)