Looks like there is a plugin for the markdown processor that would enable webm videos, I'm just not sure if doing so is a good idea.
Do we really want embeddable videos on Fiction Branches? And if I'm adding webm embeds, what about from YouTube, Vimeo, or, as I'm sure would be popular, any of the various free porn sites? That sounds like a rabbit hole I'd rather not fall down.
If more users voice support for embedded videos then I might reconsider, but for now I think we should stick with images. If you can't find a reputable/embeddable link, and you're not comfortable rehosting the image on Imgur et al, then unfortunately you probably just shouldn't use the image.
Also, fyi, the current implementation supports webp images just fine.
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