To prevent unwanted data usage, iOS does not autoplay video. Twitter on iOS has changed its default to autoplay all video and Vines and GIFs:

…from: http://9to5mac.com/2015/06/16/twitter-ios-autoplay/

Twitter is adding autoplay to GIFs, Vines, and native videos on iOS Zac Hall June 16th, 2015

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Twitter is turning on automatic playback for certain types of media in your timeline through an update to its iOS app and its website, the social network announcedhttps://blog.twitter.com/2015/introducing-a-more-seamless-video-experience-with-autoplay.

Starting today, users will notice that native videos, GIFs, and Vines all autoplay directly in the timeline when browsing Twitter without the need to tap or click to initiate playback. Sound playback for native videos and Vines will still require tapping or clicking to play to avoid hijacking the experience. Users can also initiate full playback by rotating the iPhone to landscape mode when viewing an autoplay video.

Not everyone is a fan of autoplaying media, of course, so Twitter says it will provide users with an option to disable the new feature and use the previous click-to-play behavior. Data consumption is also a concern for users, of course, so Twitter promises a WiFi-only option is available. Twitter says it will actually be opting some users out of the new feature as well to avoid issues: “If you’re somewhere with high data rates or you have low bandwidth on your device, we’ll opt you out of autoplay to avoid unexpected charges or slow performance.”

Twitter says the new feature is rolling out to its official iOS apphttps://itunes.apple.com/us/app/twitter/id333903271?mt=8&at=10laZc and the web starting today with support in its Android app coming soon. No word on if third-party Twitter apps can or will want to support video autoplay, although GIF autoplay seems harmless and useful. the social network also recently said that it’s removing the 140 character limit on direct messageshttp://9to5mac.com/2015/06/11/twitter-dm-limit-ceo-change/ sent between users soon.