To be fair, a lot of people they might assume that the target attribute can ONLY take a few discrete magic values if they haven't worked with multiple frames on

To be fair, a lot of people they might assume that the target attribute can ONLY take a few discrete magic values if they haven't worked with multiple frames on one document, so it's an obvious question to ask why it HAS to start with an underscore. (_blank has the same uniqueness as blank, if you assume the ONLY options are _blank and _self, the only commonly used values in modern web development.) Though... yeah it's pretty unsatisfying of an answer generally.