Sorry on the internet I don't know if this is sarcasm or not.
IMO I would focus on what Trisquel can do rather then what it can't. Now this is just plain marketing but I would say if asked that we can watch videos like on [https://trisquel.info/en/wiki/play-videos-without-using-flash youtube using webM without flash]. Then if users come to the message board/IRC asking about it we can explain why flash is bad (plenty of reasons). But when advertising I don't think it is good to advertise what you can't do but focus on what you can.
