AVI is the veteran of desktop video: dependable in 1995, cranky about everything invented since. It was born in 1992, Microsoft, and its real weak spot is modern life: no proper streaming, no subtitles, no love for modern codecs.
WEBM is video the way browsers like it: open, royalty-free and built to stream. A Matroska-based container restricted to VP8/VP9/AV1 video with Vorbis or Opus audio. That makes it the right destination when what you need is embedding on the web without licensing worries.