Usually anything where the BIOS can find and boot Grub itself but Grub can’t finish the boot chain for whatever reason, like /boot being on a separate partition and your main partition gets corrupted or something

It’s a monumental effort really, building a browser engine from scratch and taking it to daily driver usable is probably among the most difficult programming challenges. It’s way easier to build a new Linux kernel from scratch than a browser engine lmao

Even Microshit tried and gave up because it was so hard