I have a video with a portrait aspect ratio. (It was originally meant to be a Pixiv ugoira and ended up way too big, but it's too late to change it.) I uploaded it privately (unlisted) to check how it would look here, and the site layout doesn't handle it. It seems to fit horizontally, so the video ends up being two pages tall and unviewable unless you scroll down to find the video controls and fullscreen it.
If an admin wants to take a look, check the video on my account. (I'd rather not post the video publically while it's breaking the site layout, so I don't want to post the link here.) Looking at the CSS, a quick fix seems to be to add "max-height: 90vh" to the .node-video.node-full video rule. That way it won't scale the video bigger than 90% of the viewport height.
I could letterbox the video as a workaround, but it seems like an easy fix.