You should not add these in card templates, as they won’t, among other things, be counted as used media. Instead, you should add them directly in the fields.
Weird… you can always prepend these files with an underscore to prevent them from being considered unused (ie call them _video.webm), although this is more of a workaround than a real solution, since if you ever actually ditch that <video srv="video.webm"></video>, checking media won’t detect you don’t use video.webm anymore.
Anki does not officially support HTML video tags, and the media check does not currently recognize them. If you use [sound:video.webm], it should work.