Upload the WebM file and the converter samples frames (default 15 fps for GIF, 24 fps for animated WebP / APNG), encodes them into an animated GIF, and applies palette quantization tuned for the target (GIF GIF uses an adaptive 256-colour palette; animated WebP supports full 24-bit colour with alpha).
How short or long should my WebM be for a good GIF?
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Animated GIF is best for clips under 10 seconds. Longer than that and the file balloons (GIF is essentially uncompressed-per-frame); for longer animations target GIF = WebP or APNG, both of which compress similarly to modern video codecs and stay reasonably sized.