Links and Webs - HTML and XHTML: The Definitive Guide - page 345

The region defined by an <area> tag acts just like any other hyperlink:
when the user moves the mouse pointer over the region of the image,
the pointer icon changes, typically into a hand, and the browser may
display the URL of the related hyperlink in the status box at the bottom
of the browser window. [*] Regions of the client-side image map not
defined in at least one <area> tag are not mouse sensitive.
[*] That is, unless you activate a JavaScript event handler that writes the contents of the status box.
See the onMouse event handlers in section 6.3.1.4 .