Per the HTML 5 Spec:
"The target attribute, if present, must be a valid browsing context name or keyword. It gives the name of the browsing context that will be used." (see https://html.spec.whatwg.org/dev/links.html#links you'll need to scroll down a bit to see this.)
Futhermore "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."
So to answer your question, as I read the spec, "_new" is not a valid browsing context name as it is not one of the four reserved keywords that can begin with an underscore. Use "_blank" instead.
As always the MDN web docs have a useful discussion with examples. See https://developer.mozilla.org/en-US/docs/Web/HTML/Element/a
