I run SmugMug, a popular photo & video sharing service with over 35M visitors a month, and I just implemented Twitter Cards last night.
Since we support video, we have Player cards pointing at H.264/AAC videos in MP4 containers, but I was hoping there might be a way now, or in the future, to also point them at M3U8 playlist files as well, especially for mobile clients. That allows us to, among other things, selectively pick the right bitrate & resolution on-the-fly, allow realtime scrubbing, etc. They’re very widely supported (every iOS device, every video streaming service, etc) and provides a much better experience for your mobile clients.
Example Twitter Player Card: http://cmac.smugmug.com/Photography/Jackson-Hole/18570755_WwTXSf/1479711694_VbTbHNC