Believe it or not, using arenas does not provide free memory safety. You need to statically bound allocations to make sure they don't escape the arena (which is exactly how arenas work in Rust, but not Zig). There are also quite a lot of ways of generating memory unsafe code that aren't just use after free or array-out-of-bounds in a language like Zig, especially in the context of stuff like DOM nodes where one frequently needs to swap out pointers between elements of one type and a different type.