What is GIF?
GIF is thirty-eight years old and refuses to die, because it animates and everything opens it. The cost is its palette: 256 colours per frame, so photographs come out visibly banded. Transparency is on-or-off per pixel, with no partial opacity.
What is WEBM?
WebM is the web's own video container - Matroska slimmed down to the royalty-free codecs (VP8, VP9, AV1). Browsers play it natively and screen recorders love emitting it; the rest of the world's video tooling is distinctly lukewarm about it, which is why it so often needs converting.