The HTML5 video tag supports having multiple video sources so you can still keep the webm as one alternative and browsers will pick either the webm or mp4 depending on support.
My comment originally only said that webm was not supported prior to iOS 15. But webm videos are not fully supported in iOS 15 either afaik, so I updated the comment to specify that they started adding support for parts of it. You may have seen an earlier version of my comment without that qualifier, in which case I apologize for any confusion.
https://twitter.com/hyperchroma/status/1464656712544174083
I'm not sure if the trailer loading is an issue on the site - could you check the console for me and see what it says?