A few days ago I posted a webm file (https://e621.net/posts/3055177) but it was deleted, with the reason given being "Broken/corrupted file. (Anamorphic)"
Did I do something wrong with the formatting? It worked fine on my Chrome browser and on my iPhone.

The janitor Mairo who deleted your WEBM explained the anamorphic thing here recently.

There is a good explanation of the concept in topic #31441, in which a situation similar to yours happened.