It is hard to design. There should be some sort of precision and rounding mode control throughout calculation, meaning there should be some sort of implicit state or those controls should be sprayed into every operation. The latter is explicit but tedious [1]. The former will still surprise people from time to time. IEEE 754 binary number is sort of working well in this aspect.

[1] See QuickJS's BigDecimal extension for example: https://bellard.org/quickjs/jsbignum.html#Properties-of-the-...