You can just interpolate bits of Javascript inside C++ to access the DOM, like so:

  #include <emscripten.h>

  void sayHello()
  {
    EM_ASM(
      let foo = document.getElementById('foo')
      foo.innerHTML = 'hello';
    );
  }

See https://emscripten.org/docs/porting/connecting_cpp_and_javas...