FFmpeg is free, open source, and supports converting to and from a huge variety of formats: https://ffmpeg.org

I just tested and can confirm it can convert a gif to webm. Can't test alpha support at the moment.

It is a command line tool, though.

As an aside, if you want something more gif-like rather than video-like in behavior but still higher quality and with proper alpha support, animated pngs (apng) might be a viable option: https://en.wikipedia.org/wiki/APNG

Came here to support this answer! Apparently, ffmpeg automatically detects the alpha channel (that you'd have in your AVI/Gif) and would automatically port it into Webm using VP9. https://stackoverflow.com/a/39112195

apng is such an underrated format. Can be animated like gifs, except there are no color limitations and it supports all levels of semi transparency just like a standard png, so it can be way higher quality. I highly recommend it if you're able to use it Mirage, ezgif.com also allows you to make and convert to apng easily.

I'd also go for ffmpeg. Tho it's not as simple to use as "upload file, download file", tho it's all done locally, no need to upload any files.

FFmpeg is a Command Line Utility, but you can make a script to automate the process ^^

I'm getting help with that one, seems to be the indicated to do the job, thanks!

The site i had in mind was https://ezgif.com/ but opon further inspection it doesnt seem to do WebM, it does convert gif to WebP or as mentioned above by :tea_x: APNG with transparency

Thanks for the suggestion, I do need specifically WebM though. But seems the other one suggested above is enough.

ffmpeg can do everything you want but its all commandline (its not that scary!)
Their site here https://trac.ffmpeg.org/wiki/Encode/VP9 explains all the junk for converting to webm

There's not much point in converting a gif though, the quality loss already happened?

Thanks!

I agree with the others saying ffmpeg, it's awesome and there's pretty much nothing it can't do!
That being said, you could also look for other alternatives, like using animated WebP files! (WebP, not WebM) They support transparency and lossy compression, and you can convert APNGs to it easily with websites like ezgif.

I need specifically Webm, however, the ffmpeg seems to do the job just fine, thanks!