`<area>` with no alt text should be an error when `href` attribute is present

For example, given this HTML:

<map name="foo"><area href=""></map>

HTML-Validate does not consider it an error but should do so as per the spec:

If the area element has an href attribute, then the area element represents a hyperlink. In this case, the alt attribute must be present. It specifies the text of the hyperlink.

With the following exceptions:

The alt attribute may be left blank if there is another area element in the same image map that points to the same resource and has a non-blank alt attribute.

If the area element has no href attribute, then the area represented by the element cannot be selected, and the alt attribute must be omitted.