What are "runflats"?
Runflat tires are built with a freakishly strong sidewall designed to support the weight of the car without any air pressure. They been around for a couple decades, usually as OEM-designed solutions for vehicles that come without spare tires. They’ve been a long-standing requirement for BMW, Corvettes, AWD Siennas, and a few others. Tesla is the first manufacturer I’ve seen without a spare that doesn’t come with runflats or a tire repair kit.
The upside is that they are capable of going 50 miles at 50 mph without any air. Downside is limited options, usually only one choice in a given size, and they are a bit heavier than conventional tires. The OEM Michelins are 24 pounds each, and the drive guard runflat is 30 pounds. So bit more unsprung weight, and the stiffer sidewall makes it ride stiffer. They do seem to be a bit quieter, though.