Nothing an author can do can choose to open in a new tab instead of a new window; it is a user preference. (Note that the default user preference in most browsers is for new tabs, so a trivial test on a browser where that preference hasn’t been changed will not demonstrate this.)
CSS3 proposed target-new, but the specification was abandoned.
The reverse is not true; by specifying certain window features for the window in the third argument of window.open(), you can trigger a new window when the preference is for tabs.
