If you paste a YouTube or other website video link on a card and then click card menu—>Cache Media, the AO server will cache the video for you as an attachment on the card. This makes it possible to watch the video without visiting the original host website and potentially revealing your identity to them, and also makes it easy to download the full file. This feature relies on youtube-dl, an open-source project.
There is currently no progress bar on video caching, and depending on the size of the video and the AO server's internet connection speed, it may take several minutes or a few hours to cache a video. When the video is done caching, it will display immediately.