All currently supported compilers provide C++ language extensions for tightly packed structures (and thus non-naturally aligned structure members): MSVC via #pragma pack; GCC, Clang and emscripten via attribute((packed)).
Emscripten discourages their usage.
However, none of these compilers are able to detect and warn about all cases in which pointers to unaligned members escape a scope where the unanlignedness is known to the compiler. This causes crashes (or really slow software fixup traps) on any architecture that does require aligned memory access (old ARM, MIPS, SPARC).
It is also generally desireable to (at least optionally) express as much as possible in fully standard conforming C++.