How to render an image marked USEMAP when the image is not displayable.
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   1. Render the IMG's alt text in the place of the image.

   2. Make sure the IMG element is not styled specially (dep bug
      11011) (unless, of course, it is in a link).

   3. If the related MAP element contains only AREA elements, then:

      1. Render all the AREAs inside the MAP element with this style:

         AREA { display: list-item; }
         AREA:before { content: attr(alt); }

      2. Optionally, if dynamic context menus are not beyond the UI
         team:

         1. Override point 2 above and style the IMG element like a
            link again (dep bug 11011).

         2. When someone clicks on the IMG alt text, generate a menu
            from the 'alt' texts of all the AREA elements of that
            IMG's MAP. Display the menu.

         3. When someone selects one of the menu items, go to the URI
            selected by the relevant AREA's href attribute.

   4. Make sure that AREA elements act as links (and count as :link
      and :visited elements (dep bug 6306)).

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bug dependencies refer to http://bugzilla.mozilla.org/ entries