How do I display the value of an attribute as generated content when it has a namespace identifier?

For

<element xml:id="r123"/>

I have the style

element {content:"ID: "attr(id)}

which seems to produce:

I also tried with the namespace

element {content:"ID: "attr(xml:id)}

which produces nothing at all, not even the fixed content.

Namespaces in CSS have to be defined before they are used:

@namespace xml url("http://www.w3.org/XML/1998/namespace");

element {content:"ID: " attr(xml|id)}

Note that CSS uses the vertical bar '|' character as a namespace separator instead of the colon character ':', as colon is already used for other things.