A container packages encoded video, optional audio and timing or metadata into one file. WebM commonly carries VP8, VP9 or AV1 video and is supported by many browser playback environments.
Transparency depends on the codec profile, encoder and browser rather than the WebM extension alone. Web delivery should also consider file size, dimensions, edge appearance and fallback behavior in the target browsers.
ELI5
WebM is a type of media file built for the web. It acts like a box that holds compressed video, optional audio and timing information that a browser can play.
For example, a VP9 animation with transparency can be placed inside a WebM file for a website overlay. The file name alone does not guarantee transparency, so it must be tested in the browsers and page layout where it will appear.
