[jcelerier]

> strings are ~4 instructions (test for null terminator, output character, branch back two).

that's C strings. You also need to handle (size, data) strings like std::string_view

For the record, here's what fmt allows (from the docs):

    fmt::print(fmt::emphasis::bold | fg(fmt::color::red)
             , "Elapsed time: {0:.2f} seconds", 1.23);
    fmt::print("Elapsed time: {0:.2f} seconds"
             , fmt::styled(1.23, fmt::fg(fmt::color::green)  fmt::bg(fmt::color::blue)));
 
    fmt::print("{}", fmt::join(std::vector<int>{1, 2, 3}, ", "));

    fmt::print("strftime-like format: {:%H:%M:%S}\n", 3h + 15min + 30s);

you really think you can replicate that in 50 bytes?