Both the ball carrier and their opponent have responsibilities to avoid head-to-head contact. A ball carrier must not lower their body height or dip down just before contact with an opponent as this will risk their being in contact with the tacklers head.
Sanctions
A ball carrier may be liable to sanction if they lower their head below hips or dip down just before contact as this will risk their head being in contact with the opponent's head.

Roadmap
Law Variation
Tackler Responsibilities
Further Detail