See http://htmlhelp.com/reference/html40/special/a.html
The TARGET attribute is used with frames to specify the frame in which the link should be rendered. If no frame with such a name exists, the link is rendered in a new window unless overridden by the user. Special frame names begin with an underscore:
* _blank renders the link in a new, unnamed window
* _self renders the link in the current frame (useful for overriding a BASE TARGET)
* _parent renders the link in the immediate FRAMESET parent
* _top renders the link in the full, unframed window
In HTML 4, the TARGET attribute value is case-insensitive, so that _top and _TOP both have the same meaning. However, most browsers treat the TARGET attribute value as case-sensitive and do not recognize _TOP as having the special meaning of _top.