If there is QWebView which is partially or entirely behind another QWidget with a transparent background, calling QMediaPlayer::setVideoOutput(QVideoWidget*) with the QVideoWidget being on the same window causes the QWebView sections under the transparent widgets to turn entirely black.

It does not matter if the QWebView has a url loaded or what type of widget the transparent widget is. The QMediaPlayer also does not necessarily need to be playing or not playing any media.