Yes, which outsources the problem to Charlie. Charlie has to firewall and lockdown that browser as much as possible to reduce the risk of Alice pwning Charlie,

Yes, which outsources the problem to Charlie. Charlie has to firewall and lockdown that browser as much as possible to reduce the risk of Alice pwning Charlie, else Charlie's server could (among other things) be made to taint all output files. Browsers are not magically immune, they also just call decoder libraries.