The tag alias #76932 mp4 -> animated is pending approval.

Reason: MP4 is part of "animated"

Note: This is not the current behavior of the webm tag. I think the reason that gif is aliased is because people colloquially treat "gif" as synonymous, which people don't do as much with webm or mp4 ("Hey, you see this GIF?" versus "Hey, you see this MP4?" - people just say "video"). I think that we would need to talk about whether both webm/mp4 need to be aliased to animated or if neither do. You can also search for these files specifically with type:webm or type:mp4, so the tag is mildly redundant, albeit convenient.

The animated tag already gets added automatically on upload if the file is in either webm or mp4 format.
This alias isn't really necessary, but it wouldn't harm anything either.