To me this reaction from the Chromium devs is missing one of the most fundamental issues. I'm not fundamentally against the declarative API because of technical limitations; I am against it because it is a strong innovation lock. The current extension ecosystem is flexible enough to allow hundreds (maybe thousands) of people to actively work on privacy-enhancing extensions (ad-blockers, anti-tracking, etc.) and the technologies, heuristics, solutions to protect users' privacy on the Web are constantly evolving. The APIs are not used today the same way they were used 2 years ago. If Chrome decides to "freeze" the blocking capabilities of the browser into a declarative API that no one but Chrome devs can improve, they will be preventing people from finding new solutions to tracking and advertising (at least from extensions). It does not matter if they replicate 100% of the capabilities of today's ad-blockers, as long as it does not allow evolution and adaption it will become obsolete. There is precedent in this matter: Safari also has a similar API and it has been a huge pain for ad-blockers developers. The reason is simple: Apple or Google do not have the same strong intensives that we have to continuously improve the blocking capabilities of the user agent. My fear is that this declarative API will be an ok-replacement for today's content blockers, but will not allow the same kind of fast paced development we benefit from today in the space of privacy extensions.