WTV only plays in Windows Media Center and a few media players. MKV can carry several audio and subtitle tracks in one file. A WTV file usually holds MPEG-2 or H.264 video with AC3 or MP2 audio. WTV is at home in recordings made by Windows Media Center; MKV is the usual choice for films with several audio and subtitle tracks.
The defaults suit most files. Under Options you can lower the resolution, trade quality against file size, remove the sound and trim the start and end. 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 WTV 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.