Problem in brief

Implementation of DateTimeFormat.formatToParts() is missing.

Details

Any attempt to add this method will likely need to look at the implementation of options which the DateTimeFormat constructor currently ignores.

options is most likely integral to the implementation DateTimeFormat.formatToParts() and as such fixing this issue will probably be involved.

Supporting options will also affect DateTimeFormat.format().

Here is a possible test for the latter:

var options = {
  weekday: "long",
  year: "numeric",
  month: "long",
  day: "numeric",
};
var dateTimeFormat = new Intl.DateTimeFormat("en-US", options);

// Chrome: Wednesday, May 24, 2023
// HtmlUnit: 05/24/2023
console.log(dateTimeFormat.format(new Date()));