You can create a BIOS bootable CDROM using grub-mkrescue[1]:

  (mkdir steamos && cd steamos && unzip ~/Downloads/SteamOSInstaller.zip) && grub-mkrescue -o steamos.iso steamos

It boots with QEMU:

  truncate --size 20G /var/tmp/disk.img && qemu-system-x86_64 -cdrom steamos.iso -hda /var/tmp/disk.img,if=virtio -vga std -m 2048 -enable-kvm

[1] https://www.gnu.org/software/grub/manual/html_node/Making-a-...

As a requirement, no, but basically every new machine (Macbooks \w Mac OS X, Chromebooks \w Chrome OS, Windows RT tablets, Windows 8 machines) ships booting off of UEFI.