MTI stands for Mandatory To Implement. It is a term used in the context of WebRTC to indicate which voice and video codecs are mandatory to implement to be WebRTC compliant.
For voice, the MTI codecs are G.711 and Opus, along with DTMF. This is specified in RFC 7874.
For video, the MTI codecs are H.264 and VP8 (web browsers and devices must implement them both, while applications and services can decide which ones to use). This is specified in RFC 7742.
Web browsers usually have VP9 support as well (and sometimes AV1), even though it is an optional video codec in WebRTC.
