Based on the product spec sheet[1] it appears that the Framework Desktop motherboard is mini-ITX, and that the chassis is[2] as well.

It’s fully possible that I’ve missed something, but it doesn’t appear to be proprietary to me.

[1]: https://frame.work/desktop?tab=specs

[2]: https://frame.work/products/desktop-case

This is correct, we used a standard Mini-ITX form factor, and we've seen a number of customers pick up just the Mainboard and drop it into standard ATX cases. The power supply is also a standard FlexATX PSU.

I’m unable to pass the human test on those links but if it’s true mini-ITX including standardized USB and IO headers that’s pretty cool!