The xml:base attribute annotates an element with a base URI value.

Example:

<graphic xml:base='http://www.milowski.com/thesis/' src='issac-scarfex.jpg'/>

Rules for the base URI of an element:

1. The value of the xml:base attribute.

2. The base URI of the parent element if there is a parent element.

3. The base URI of the document.

Keep in mind that the 'xml' prefix is pre-defined to be 'http://www.w3.org/XML/1998/namespace'.