Under the Visa Waiver program, can I leave by land in transit to Guatemala? [duplicate]

When entering the US by air, can one cross the Mexican border with the intent of leaving Northern America? How does the transit time in Mexico count toward the 90 day limit of the VWP, and how does the US register this exit?

The only official reference I can find on the CBP website is that they will not always properly register such an exit and burden of proof is on the traveler, but that page lacks detail.