https://c9x.me/compile/ (c frontend: https://github.com/michaelforney/cproc)
https://github.com/sysprog21/shecc (this fundamentally targets risc-like architectures, but is very clearly written).
perhaps eventually a simple well-defined semantics will emerge (i think projects like the above already support more of a regehr-style c rather than iso).
"SubC is a fast and simple public domain compiler for a clean subset of the C programming language. It can compile itself and passes gcc -Wall -pedantic. Its internals are described in detail in the book Practical Compiler Construction. Note that SubC is a teaching vehicle rather than a production compiler."
https://t3x.org/subc/index.html
And when I say toys, I don't mean mean plastic, trivial items. I mean, I want to buy all of his books, download all of his tools, and collect the whole set, like Legos or other toys never did for me when I was a child.