GIF is limited to 256 colours and produces very large files for anything longer than a few seconds. WebM is royalty-free and well suited to embedding in web pages. GIF is at home in short looping clips and reactions; WebM is the usual choice for royalty-free video on websites.
This is demanding work for a browser: a long or high-resolution file can take several minutes, and your device does all of it, so keep the tab open.
This pair uses the compatibility engine, ffmpeg built for the browser, because GIF is not handled by the browser's built-in codecs. It is a one-off download of about 10 MB that the browser keeps, it is slower than the built-in codecs, and it takes files up to 1 GB.