As an intervention we want to limit the rate at which timers in out-of-view, cross-origin frames are able to fire. Until very recently it wasn't possible for script authors to easily determine whether their content was visible to the user or not. A common pattern therefore is to use a continuous setTimeout() loop for driving animations without considering visibility, which can be very costly for performance.
Docs: https://docs.google.com/document/d/1Xa8sd2M_Eh6hGSWeKTy_akzKdxcVZjCk4PQMqV8yXXw/edit https://github.com/WICG/interventions/issues/9