I currently have to use this CSS overload with the Stylus extension to avoid having videos (and also gifs since they are converted to mp4 videos by Lemmy) stretched to fill the whole page width:
video {
width: 25vw !important;
}
The !important is there because the HTML uses a 100% width for videos:
(I have to use a screenshot here since Lemmy now filters any HTML code away even if it's inside code tags)