What are namespaces?

In order to allow data owned by two different DTDs to live together in a single XML document, it is possible to prefix the tags by a name referencing the DTD involved.

You need first to declare the namespaces that will be used in parent element beginning tag:

<para xmlns:xhtml='http://www.w3.org/TR/xhtml1'>

Then, inside this element, you will be allowed to use the "xhtml:" prefix for tags which do not belong to the presentation DTD but to the XHTML namespace:

<para xmlns:xhtml='http://www.w3.org/TR/xhtml1'> This point is <xhtml:b>very</xhtml:b> important! </para>