Now that I have searched almost the whole internet, I ask if someone can help.
To the point: The video image of LEO is a MJPEG stream. The should be recorded by a recording button. Another button is to stop recording and display as a preview in a window (on the LEO UI). Quasi buffered. If the recording is OK, you can download it or delete it with a button. A new recording should overwrite the old one.
Actually everything as in the WEBRTC example of UV4L. I once adapted the example and fit into my UI. Everything works, but I can not communicate with ROS anymore. No idea why.