HTML Guide
The element X requires an attribute Y, but it’s not present. Check that element and add the required attribute, like in <X Y="value">.
Related W3C validator issues
The src attribute for <img> tags is required, to define the source of the image, like in this example:
<img src="photo.jpg" alt="wombat" /><img> tags used to display images require the attribute src to indicate the source of the image, for example <img src="/img/photo.jpg" />.