WordPress does not want to default to WebM out of the box.
Since I did a blog about WebP minus well do one about WebM. As some of you may know that I run a hybrid video/regular blog we use WebM because it’s the latest and greatest. Back when I was running with the Angular theme that included its own version of MediaPlayer.js and all was good because the WebM files were being referenced first and foremost. That changed when I reverted back to a more standard WordPress theme that relied on the out-of-the-box MediaPlayer.js which is totally fine except for one problem.
Default WordPress HTML5 video player grabs MP4’s first. Not WebM.