MPG does not play in browsers or on phones and is far larger than MP4. WebP is smaller than JPG or PNG at similar quality and keeps transparency. MPG is at home in DVDs, Video CDs and old camcorder footage; WebP is the usual choice for images on websites. WebP has no sound, so the audio track is dropped.
The defaults suit most files. Under Options you can set the frame rate, set the width, trim the start and end and set the quality. 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 MPG 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. The result is a looping animation of at most 30 seconds, counted from the start point you set under Options. The defaults are 12 frames per second and 480 pixels wide. Unlike a GIF it keeps full colour, and it has no sound.