There is a note alluding to special (non-terminating) error handling suitable for HTML forms and URLs, but it only says that one or more ASCII characters will be emitted in lieu of an unrepresentable character.  HTML5 requires decimal character references at least in some cases, but the note suggests that there is more to it.  Perhaps it is worth including more details or a reference to where such can be found.  The encoding specification might even be the right place to define some of this, but that is a separate issue.