It seems like there’s hidden rules on how to correctly get something to embed properly from giphy, imgur, or redgifs. I have some control over it, but most of the power lies in the Discourse software or the linked websites themselves.
Here, I tried plugging in some links to see what works. Note that Discourse does some background processing, so an embed might appear to work initially, but ~10 minutes later your embed could break.
This is also dependent on some technical details of the website you’re linking to, so they could change their behavior in the future. Also the file size of your link could impact preview as well. Overall, it seems that problems are most common when linking directly to a .gif file
If problems keep persisting, I could probably allow for iframe embedding from these sites.
Not allowing edits on this topic for now
Giphy
page url
https://giphy.com/gifs/nfl-sports-football-sport-J2sJ5TqvdYhqNucBrA/links
pasting the short link
https://gph.is/g/Evex1wg
gif link (downloaded locally, so this will not preview with larger files)
https://media.giphy.com/media/J2sJ5TqvdYhqNucBrA/giphy.gif
html5 video link
https://giphy.com/gifs/J2sJ5TqvdYhqNucBrA/html5
iframe
media source
https://media.giphy.com/media/J2sJ5TqvdYhqNucBrA/source.gif
media social (downloaded locally, so this will not preview with larger files)
https://media.giphy.com/media/J2sJ5TqvdYhqNucBrA/giphy.gif
media mp4
https://media.giphy.com/media/J2sJ5TqvdYhqNucBrA/giphy.mp4
media small (downloaded locally, so this will not preview with larger files)
https://media.giphy.com/media/J2sJ5TqvdYhqNucBrA/giphy-downsized.gif
imgur
copied link, which is the same as the page url
https://imgur.com/gallery/0cbxBGx
https://imgur.com/gallery/0cbxBGx
link to video location source
https://i.imgur.com/avLOBQ8.mp4
imgur link without /gallery/. I think this is how they treat nsfw links?
https://imgur.com/0sNaOow
https://imgur.com/0sNaOow
above link to video source
https://i.imgur.com/0sNaOow.mp4
above link to gif
https://i.imgur.com/0sNaOow.gif
redgifs
page url
https://www.redgifs.com/watch/negligibledeficientauklet-small-tits-blonde-asian-boobs
https://www.redgifs.com/watch/negligibledeficientauklet-small-tits-blonde-asian-boobs
share link
https://redgifs.com/watch/negligibledeficientauklet
iframe
direct gif link (small) (downloaded locally, so this will not preview with larger files)
https://thumbs.redgifs.com/NegligibleDeficientAuklet-small.gif
direct gif link (large)
https://thcf8.redgifs.com/NegligibleDeficientAuklet-size_restricted.gif
https://thcf8.redgifs.com/NegligibleDeficientAuklet-size_restricted.gif
right click copy video location
https://thcf8.redgifs.com/NegligibleDeficientAuklet-mobile.mp4