"an optional argument value"

[Ms2ger]

When the Intl.RelativeTimeFormat.prototype.format is called with an optional argument value, the following steps are taken:

When the Intl.RelativeTimeFormat.prototype.formatToParts is called with an optional argument value, the following steps are taken:

Both of these methods have two required arguments, rather than a single optional one.

[littledan]

That's right, this was some poor copypasta. PRs welcome.