Why a WebM is a fraction of the GIF size
GIF stores every frame with a 256-colour palette and compression designed in the late 1980s, with no idea that consecutive frames resemble each other. WebM uses a real video codec that spends its bits on what changes between frames, so the same clip routinely comes out several times smaller, sometimes by a factor of ten, while gaining back full colour.
The conversion runs in your browser: the GIF frames are decoded locally and re-encoded as VP8 video in a WebM container, with nothing uploaded along the way.
- Drop the GIF; frames decode on your own device
- A real video codec re-encodes the animation as WebM
- Full 24-bit colour replaces the 256-colour GIF palette