Many avatars already come with face tracking in the form of unified expressions.

Unified expressions likely needs no introduction, however it is a standardisation of parameter values, face, eye and neck shape keys.

Using these parameters if available on an avatar will significantly increase the liveliness of the avatar while being captured by selfie expression.

These values can be found at https://docs.vrcft.io/docs/tutorial-avatars/tutorial-avatars-extras/unified-blendshapes