XML in a Nutshell, 3rd Edition

Setting a Default Namespace with the xmlns Attribute

You often know that all the content of a particular element will come from a particular XML application. For instance, inside an SVG svg element, you’re only likely to find other SVG elements. You can indicate that an unprefixed element and all its unprefixed descendant elements belong to a particular namespace by attaching an xmlns attribute with no prefix to the top element. For example:

<svg xmlns="http://www.w3.org/2000/svg"
     width="12cm" height="10cm">
  <ellipse rx="110" ry="130" />
  <rect x="4cm" y="1cm" width="3cm" height="6cm" />
</svg>

Here, although no elements have any prefixes, the svg, ellipse, and rect elements are in the http://www.w3.org/2000/svg namespace.

The attributes are a different story. Default namespaces only apply to elements, not to attributes. Thus, in the previous example, the width, height, rx, ry, x, and y attributes are not in any namespace.

You can change the default namespace within a particular element by adding an xmlns attribute to the element. Example 4-4 is an XML document that initially sets the default namespace to http://www.w3.org/1999/xhtml for all the XHTML elements. This namespace declaration applies within most of the document. However, the svg element has an xmlns attribute that resets the default namespace to http://www.w3.org/2000/svg for itself and its content. The XLink information is included in attributes, however, so these must be placed in the XLink namespace using explicit prefixes. ...