ABSTRACT. Scripts on webpages could steal sensitive user data. Much work has been done, both in modeling and implementation, to enforce information flow control (IFC) of webpages to mitigate such attacks. It is common to model scripts running in an IFC mechanism as a reactive program. However, this model does not account for dynamic script behavior such as user action simulation, new DOM element generation, or new event handler registration, which could leak information. In this paper, we investigate how to secure sensitive user information, while maintaining the flexibility of declassification, even in the presence of \emph{active attackers}---those who can perform the aforementioned actions. Our approach extends prior work on secure-multi-execution with stateful declassification by treating script-generated content specially to ensure that declassification policies cannot be manipulated by them. We use a knowledge-based progress-insensitive definition of security and prove that our enforcement mechanism is sound. We further prove that our enforcement mechanism is precise and has robust declassification (i.e. active attackers cannot learn more than their passive counterpart).