X9 boards with recent BIOS have UEFI support. Because BMC/IPMI firmware on Supermicro is sort of tied to BIOS (in that you shouldn't be too far apart in versions), if there were a modern HTML5 interface on the BMC, you'd have to have a recent BIOS with UEFI support.
That said, you can disable pretty much all of UEFI on SM motherboards, so if you get a recent one with HTML5 interface, you could easily disable UEFI. The only UEFI-related bit I can't disable on my X11SRA (no IPMI) is the built-in UEFI shell as a boot option. That shows up even if you select "Legacy" as the boot mode.
I really like the recent SM BIOS as you can literally use both UEFI and legacy at the same time. For example, you can have some slots use the UEFI ROM, and others use the legacy. And, you can have both modes enabled for boot, so it's easy to stick in a USB drive, hit F11 for the boot menu and boot, regardless of how the USB is formatted. No need to switch between modes.