WhatsApp is testing in-app YouTube video playback in the iOS version of their beta app.
A hidden feature in version 2.17.40 of the app, uncovered by WABetaInfo, allows users to launch playback of links to YouTube video sent in messages in a picture-in-picture window. Users are able to resize the floating window using pinch to zoom, open the video in a full-screen dedicated page, and also hide the window without interrupting playback by docking the video on the size of the chat.
Users are however not able to continue playback when entering other apps. The feature is supported on all versions of the iPhone above iPhone 6, but not iPhone 5, likely due to the smaller screen, and it is not clear if the feature will also be coming to Windows Phone and Android.
Barring any major issue, the feature should come to the mainstream version of the app in a number of weeks.
See it in action in video at WABetainfo here.