What this says, in case it's not obvious, is that you can't get away with
<img src='madonna.jpg' src='child.jpg'>
One of the nice side-effects of this rule, for a programmer, is that all the
XML processors and libraries have a function that, given an element and the
name of an attribute, can return the value of the attribute, without having to
worry about multiple occurrences.
Of course, such a function can also signal that there was no such attribute on
that element.