example CSS for moving aspect-ratio onto media-provider inside player

style.css

/* remove aspect ratio from player and set variable for controlling top margin */
%root% {
    aspect-ratio: unset;
  	--player-controls-top: 6rem;
}

/* add aspect ratio to the media provider and set margins */
%root% media-provider {
    position: relative;
    aspect-ratio: 16 / 9;
    margin-top: var(--player-controls-top);
    margin-left: 5%;
    margin-right: 5%;
    width: 90%;
}

/* make sure buffer indicator remains centered */
%root% .xmp-buffering-indicator {
    top: var(--player-controls-top);
  	bottom: 0;
    height: unset;
}

/* force controls to remain visible & remove gradient */
%root% media-controls {
  background: none;
  opacity: 1;
  visibility: visible;
}