[html] Inputting a default image in case the src attribute of an html <img> is not valid?

In addition to Patrick's brilliant answer, for those of you who are searching for a cross-platform angular js solution, here you go:

<object type="image/png" data-ng-attr-data="{{ url || 'data:' }}">
    <!-- any html as a fallback -->
</object>

Here's a plunk where I was playing trying to find the right solution: http://plnkr.co/edit/nL6FQ6kMK33NJeW8DVDY?p=preview