A target of a link can actually be any valid string as well as the special cases.
Otherwise, a new browsing context is being requested, and what happens depends on the user agent’s configuration and abilities — it is determined by the rules given for the first applicable option from the following list:
Part of #165
A target of a link can actually be any valid string as well as the special cases.
from here: https://www.w3.org/TR/html52/browsers.html#the-rules-for-choosing-a-browsing-context-given-a-browsing-context-name