Thanks @Falco - I gave the gfycat mp4 version a try. The good thing is, the video is able to load on iOS devices. The only downside is there is no thumbnail. The appearance is exactly the same as my screenshot above. The exception - the video actually plays.
On the desktop, there is no trouble with
.webm besides Apple just flat out not supporting it within Safari. Nothing we can do about that (thanks for posting the Can I Use link @Mittineague, super helpful).
One item of positive feedback that I did receive was regarding the size of the gfycat embed within onebox. Everyone that weighed in (regarding desktop browser appearance) seemed to prefer it to the native size of the YouTube embeds within Discourse