Client-side image map data (i.e. the coordinates) is stored in HTML files (and can be embedded directly into a page containing other HTML elements). The Web browser program interprets client-side maps. When a visitor to a Web page clicks within a client-side image map, the browser processes the map data without interaction with the Web server.
Client-side image maps include the map information in the HTML document with the image map.
Since the URL's associated with the hot spots are stored in the HTML document the client has the information to process a user's click immediately.
This method does not rely on the server therefore it also allows offline viewing of Web pages, from a hard drive or CD-ROM. This technique is becoming commonly used to allow users to explore the contents of a CD-ROM via a browser.
The user will see changing URLs on the toolbar at the bottom of the screen as the pointer is moved over the image.
The disadvantage to this approach is that the implementation of client-side image maps has only been adopted as of HTML version 3.2, so not all browsers can support their use. Use of a text alternative would make links visible without seeing the image. (Figure 1)(see also: Defining Clickable Regions In An Image)
Miller (1994) purports that an image map that looks good from a designer's perspective does not necessarily mean that the functionality is good; he also claims that poorly designed image maps can cause users to spend extra time navigating down the wrong path. The author suggests design issues should be taken into account for all usage contexts; for example, it would not be appropriate to place the 'clickable' regions on the image map too close together as this would create confusion.
(Figure 1: Example of Web Page without images- Source: http://www.nhm.ac.uk/)