WEB Video webrtc always on top

I have developped a website with a webRTC videoconference. I want to add a button that allow us to minimize the video on a corner of the screen/and reversibly re-open on a normal size. And most of all and i have searched everywhere, i want that the "mini-video" stay always on top while using others windows. (like skype but on web)

languages: Java - Spring boot Angular 6

Thanks for your help