If your concern are webpages, you should first try sending SIGSTOP manually to some PID. Stopping a particular webpage may not be doable by stopping PIDs. There

If your concern are webpages, you should first try sending SIGSTOP manually to some PID. Stopping a particular webpage may not be doable by stopping PIDs. There's nothing stopping multiple processes being involved in managing multiple windows non-exclusively (i.e. not only can a process manage multiple windows, a window can be managed by multiple processes). Your web browser may be beast enough to really complicate what you're trying to do.