WebM Alpha only displayed in Chrome?

问题

I've implemented a WebM video with the video tag. The alpha channel is displayed correctly in Chrome but not in Firefox nor Edge - there's a black background.

I thought those browsers had good support for WebM? Am I missing something?

Markup looks like this:

<video autoplay loop muted poster="example.jpg">
  <source src="video.webm" type="video/webm">
</video>

Thanks in advance! :)

//EDIT I got alpha channel working with Firefox: Used VP8 instead of VP9 codec. Edge is still a mystery.

来源：https://stackoverflow.com/questions/55061630/webm-alpha-only-displayed-in-chrome