If one specifies a defaultStatus property, this specifies what the browser will display if not over a link, and this overrides the specification of the status line back to blank in the onMouseOut event handler using the status property. In the code below, I specify the defaultStatus property between <script></script> tags so this is executed when the page loads, and then the onMouseOver and onMouseOut event handlers are executed when over and off the Yahoo link. Because the defaultStatus property has already been set (when the page loads), during a mouseover of a link, we get the URL of the link, with the status line returning to that specified by defaultStatus when cursor is off the link.
Through the status property of the window object, one can specify a message to appear instead of the URL during the mouseover. However, one must disable the default action, which is the display the URL.