I'm traveling to Tijuana soon overland and will obtain an FMM as per the legal requirement. However its no clear if there's an obligation to actually print it out or if I can show it on my phone. Do the rules ever clearly state this for land crossings?
I've had a quick look at a handful of Mexico tourist help websites, and all of them mention printing your online FMM and receipts. One of them specifically says "you must print" and none of them give the impression that it can be just shown on a device etc.
You’ll receive an acceptance email that you must print and bring with you to get stamped at the border.
And Dennis has commented with a link to the official INM (Mexico Immigration) website FMM application page which also states that the form should be printed
The applicant shall print and bring the FMM to the facility aimed for the international transit of persons where he/she wishes to enter through.