XML in a Nutshell, 3rd Edition

Empty Elements

Some elements do not have any content at all. These are called empty elements and are sometimes written with a closing />. For example:

<image source="bus.jpg" width="152" height="345"
       alt="Alan Turing standing in front of a bus"
/>

These elements are declared by using the keyword EMPTY for the content specification. For example:

<!ELEMENT image EMPTY>

This merely says that the image element must be empty, not that it must be written with an empty-element tag. Given this declaration, this is also a valid image element:

<image source="bus.jpg" width="152" height="345"
       alt="Alan Turing standing in front of a bus"></image>

If an element is empty, then it can contain nothing, not even whitespace. For instance, this is an invalid image element:

<image source="bus.jpg" width="152" height="345"
       alt="Alan Turing standing in front of a bus">
</image>