i might have put this in the Tags category, but i'm suggesting new serverside code so...
video tagging is a mess, "video" itself has 600 entries and no wiki entry (for all i know it's for diegetic video), yet there's over 1000 webms and ~400 mp4s, are they not videos?
i've filed a implication request for this https://booru.allthefallen.moe/forum_topics/133
but here i wanted to discuss the merits of dropping "webm" and "mp4" entirely, and instead classifying videos based on encoding (codec)
all the videos here are displayed with a HTML5 <video> tag. i cant imagine there's any browsers that support <video> but not both MP4 and WEBM inside them.
what determines if a device is able to play a video is the codec, especially mobile devices which don't have the power to brute force software/gpu compute render codecs they lack hardware support for.
this means that searching for "webm" or "mp4" is meaningless, because it's a matter of whether your device supports the codec, which is currently invisible.
i suggest new meta tags, they will require some data from the file header to generate but nothing any number of command line tools could do (i'm sure ffmpeg will return just the codec with the right arguments)
the tag webm should be removed, all existing webms should be tagged "video" and then by codec, which will be either VP8, VP9 or AV1. this is why some WEBMs won't play for people but others will - because they have different codecs.
the same for the tag mp4, to be replaced by h264 and h265 (aliased to AVC and HEVC respectively). again lots of devices can't play HEVC, but everything can play h264.
bonus points for doing this with the AUDIO TOO - i've seen a lot of video files marked as having audio but apparently being silent, and i wonder if it's because this browser doesn't support newer audio codecs (like opus on webms)
finally i suggest aliasing h264 and VP8 to a tag like "compatible video" (thats a shit name but you get the idea) because these will play on all devices, or the inverse, tagging the newer less supported codecs as "limited support".
SUMMARY: BENEFIT FOR NON-NERDS: find videos by searching "video", and find videos which always work with "compatible video"
BENEFIT FOR NERDS: selectively -ing codecs you know your device doesn't like (i bet when AV1 rolls out more almost no phone will play it) so you can just find every video you can watch.
i know this is a long ramble, but there's a lot of confusion here about why some webm's wont play and if it was tagged "VP9" instead of webm, people would realise what the problem was pretty quickly, rather than blaming webm.