Features of Watch Later in Apple

I am using my iPad when visiting websites like Contraband and YouTube. I noticed that most of the videos are not playing in those sites. I know that iOS doesn’t have a Flash so I want to know the rules for boxed iPad app and also, what are the general rules of Watch Later feature of Apple? What videos can play and cannot be played in Watch Later feature? I want to avoid the hassle of bookmarking something to watch later, but when I go to the link it won't play. What should I do? Help please.
