So as my time passes on e621 I've stumbled across some webms that already exist as regular videos on Youtube, for example:

post #813528
a Telemonster soundtrack video, a Korean animated series

post #715492
Racist Mario, an animation by Flashgitz previously posted on Newgrounds as well

post #863503
Underpants - Normal Ending by 123pendejos, an animated parody

All of this makes me think of whether this is actually considered / acceptable on e621, or not. Reasons for that are:

-It's an easy rip off of Youtube, reposted on e621. (This is easily downloaded as webm with youtube-dl)
-Sometimes the quality of the video is decreased. (People do put up youtube as the source for better quality.)
-It may be better to use the blips or forums to post Youtube videos as links.

However:
-People who tag certain interests may find this video and appreciate being here to be watched
-Youtube videos reposted on e621 may potentially increase viewer population on Youtube
-People on e621 have their own area where you can write your comments and reactions.

My question and TL;DR to you is:
Do you find videos previously posted on Youtube that are now here as Webms acceptable?

Depends on the length of the video, and how relevant it is to the fandom in general.
Here is a good example: https://e621.net/post/show/544246

Yeah, if it's relevant to us it can stay.

As long as whoever posted it got the youtubes own VP9 webm stream, then on technical level I'm fine with this. I see people taking H264 versions, then converting them making huge files with lower quality or couple cases where container is simply renamed to .webm, please never do that. As for content itself, it's up to admins.

I personally feel that if it's relatively new content created by someone who is still getting money out of views, then it's bit douche move to rip and upload it anywhere as those videos do not generate much to begin with.
Telemonster seem to have gained this fangroup in itself but company which created them have majority behind regional restrictions.

So really depents on content. Especially when there has actually been cases where artist uploads lewd stuff to youtube as they have no idea how to handle video formats and simply hope that nobody presses that flag button on private video.