Due to the way this issue was resolved <https://bugzilla.mozilla.org/show_bug.cgi?id=869878> the phrase "The parameter's value must be 'utf-8'" may be misleading.  It ought to be changed in one of the following ways, in order of descending preference:

* Remove the phrase; or
* Change the phrase to "The parameter's value should be 'utf-8'" or
 "The parameter's value ought to be 'utf-8'"; or
* Add a sentence stating that implementations need not treat
 values other than "utf-8" as an error.