I've noticed videos will autoplay on iPhone using safari. It's been slowing eating a large chunck of my data each month. I've installed content blockers but they haven't helped. The new Safari 11 in MacOS has a setting for video autoplay but iOS does not. Several apple forumns have been no help.

https://discussions.apple.com/thread/8273797

https://discussions.apple.com/thread/8069882