code: function valuesEqual(command, val1, val2) {
          if (val1 === null || val2 === null) {
              return val1 === val2;
          }
  
          return command.valuesEqual(val1, val2);
      }
docs: This entire function is a massive hack to work around browser incompatibility.  It wouldn't work in real life, but it's good enough for a test implementation.  It's not clear how all this should actually be specced in practice, since CSS defines no notion of equality, does it?