Intl.DurationFormat instances also have several internal slots that are computed by the constructor:
...
[[MillisecondsStyle]] is one of the String values "long", "short", "narrow", or "fractional" identifying the formatting style used for the milliseconds field.
Section 1.5 from the rendered TC39 proposal says that:
(emphasis mine)
while Table 3 says that:
(emphasis mine)
This seems inconsistent and possibly incorrect.