2.2.31 [HTML] Section 16.3.2, Target semantics
User agents should determine the target frame in which to load a linked resource according to the following precedences (highest priority to lowest): If an element has its target attribute set to a known frame, when the element is activated (i.e., a link is followed or a form is processed), the resource designated by the element should be loaded into the target frame. If an element does not have the target attribute set but the BASE element does, the BASE element's target attribute determines the frame. If neither the element nor the BASE element refers to a target, the resource designated by the element should be loaded into the frame containing the element. If any target attribute refers to an unknown frame F, the user agent should create a new window and frame, assign the name F to the frame, and load the resource designated by the element in the new frame.
All Document Modes (All Versions)
If any target attribute refers to an unknown frame F, the resource is loaded in a new window. The resource is not loaded in a new frame and hence does not apply the unknown name F to a new frame.