Safari on iOS already has face detection. But I don’t think there is an API for it.

If you can’t see well and so enable VoiceOver, when a webpage opens the camera to take a photo the phone will what it sees, “like face in centre of photo”.

It’s very usefully for blind of partially sighted people to take selfies.

Here is a reference: https://www.apple.com/uk/accessibility/iphone/vision/

Edit: added link