This tool reads your MKV video directly in the browser and encodes a MP4 copy, so there is no upload, no queue and no waiting for a server. It works on desktop and mobile.
MKV files are popular for downloads and screen captures, but many TVs, phones and editors will not play them. MP4 is the safest choice for compatibility.
The browser must be able to decode the video inside the MKV. Common H.264 and VP9 files work; unusual codecs may not.