HTML and CSS Reference
One way to enable an image map is by adding the ismap attribute to an
<img> tag placed inside an anchor tag ( <a> ). When the user clicks some-
where in the image, the graphical browser sends the relative X,Y co-
ordinates of the mouse position to the server that is also designated in
the anchor. A special server program then translates the image coordin-
ates into some special action, such as downloading another document.
A good example of the use of an image map might be to locate a hotel
while traveling. For example, when the user clicks on a map of the re-
gion he intends to visit, your image map's server program might return
the names, addresses, and phone numbers of local accommodations.
While they are very powerful and visually appealing, these so-called
server-side image maps mean that authors must have some access to
the map's coordinate-processing program on the server. Many authors
don't even have access to the server, let alone a program on the server.
A better solution is to take advantage of client-side image maps.
Instead of depending on a web server, the usemap attribute for the <img>
tag, with the <map> and <area> tags, allows authors to embed the inform-
ation the browser needs to process an image map in the same document
as the image. Because of their reduced network bandwidth and server
independence, the client-side image maps are popular among document
authors and system administrators. [ Client-Side Image Maps, 6.5.2 ]