If we want to do that, we should do it now.

Spec wise this should be easy

@arskama would this be hard implementation wise?

Do we recall why we move it to the ctor?

ResizeObserver, PerformanceObserver, MutationObserver has it as part of observe:

undefined observe(Node target, optional MutationObserverInit options = {});
undefined observe(Element target, optional ResizeObserverOptions options = {});
undefined observe(optional PerformanceObserverInit options = {});