Additional package repostitories
Need qt5-qtwebengine-freeworld package from rpmfusion. qt5-qtwebchannel has something to do with something, Rex Dieter knows what, ask him.
For Dolphin video previews the package ffmpegthumbs is needed and setting it in Settings -> General -> Previews -> Video Files. It does need a process restart (ctrl+alt+esc, click the dolphin window, start it again) and when a folder with video files is opened, it creates thumbnails.