You specify alternate text for an image-map hotspot via an attribute of the hotspot <area> tag. The alternate text relates to the hotspot link destination. Unlike alternate text for the <img> tag, you cannot specify alternate text for the <area> tag with a PI marker or a paragraph format. And you might want to use the title attribute of the <area> tag instead of the alt attribute.
§32.9.1.1 Assigning alternate text to an image-map hotspot
§32.9.1.2 Using the title attribute for alternate text for a hotspot