According to the HTML5 WIP Spec:
A valid browsing context name is any string with at least one character that does not start with a U+005F LOW LINE character. (Names starting with an underscore are reserved for special keywords.)
A valid browsing context name or keyword is any string that is either a valid browsing context name or that is an ASCII case-insensitive match for one of: _blank, _self, _parent, or _top.” –Source
That means that there is no such keyword as
_new in HTML5, and not in HTML4 (and consequently XHTML) either.
That means, that there will be no consistent behavior whatsoever if you use this as a value for the target attribute.