Requested feature overview description.
Similarly to images being created 800x800 pixel restricted JPG samples, create 800x800 pixel (would turn 16:9 videos to 450p which should be useable) 30 FPS restricted VP8 and h264 sample versions of videos.

Why would it be useful?
Artists and users right now are discouraged to post the best version of the video as if it's too high quality, huge portion of userbase are not able to see it and may even crash their devices.

Using VP8 and h264 would increase compatibility with really old devices (android versions below 4.4), software (including IE with Media Foundation Components) and iDevices immensily and as they would be sample versions, quality wouldn't be an issue. This way newer and more efficient VP9 and high quality settings would be more encouraged to be used.

This would also fix problem described in forum #251840, where those who have chosen sample versions of posts (also all every guest user) don't get 4K resolutions bleed way out of their browser windows.

As video files are also largest in filesize in most cases, this would further decrease bandwidth usage of the site as well as speed up video playback and buffering to end users.

What part(s) of the site page(s) are affected?
Post viewing.
User settings for samples.

android <4.4 comprises 4.1% of total android marketshare as of july 2018 and will continue to drop
https://fossbytes.com/most-popular-android-versions-always-updated/

supporting internet explorer is never a good reason

as for iphones and ipads, apple will concede this battle in the long run when youtube drops H264 support
https://www.flatpanelshd.com/news.php?subaction=showfull&id=1485521960

ampiaxxx said:
android <4.4 comprises 4.1% of total android marketshare as of july 2018 and will continue to drop
https://fossbytes.com/most-popular-android-versions-always-updated/

supporting internet explorer is never a good reason

as for iphones and ipads, apple will concede this battle in the long run when youtube drops H264 support
https://www.flatpanelshd.com/news.php?subaction=showfull&id=1485521960

Those were only few examples given. I have been using VP9 to do encodes for well over year now because everything that should support webm should also support the codec already. We are actually now at the point of transferring to AV1, which is yet another hurdle.

However it still seems like I'm constantly bumping to users who think files uploaded here are corrupted, when reality they aren't and they are just confused because webm on derpibooru worked, where they do serve h264 sample to users. And there has been users who have been confused why only some webm files on this site work at all, where it turns out it's 4K 60 FPS video or user has android 4.1.

Because of this it does make sense to have samples that cover as wide compatibility as possible, so that "webm" mark on post isn't this monster you should be avoiding because you cannot see it anyway. Just make it work for everyone and those who can watch original can just do so, similarly to samples with regular posts. Reason why gfycat uses low quality settings and VP8 is pretty much the same, just make it work and have GIF format as last fallback.

Also as they are samples, they should technically be possible to re-generate in the future as long as the original content is untouched.