The HintContext Javadoc doesn't talk about XML
namespaces support at all, but it is clear from
the impl in DefaultContext that there is none:

public String getNamespaceURI() {
    throw new UOException();
}

For some kinds of grammar providers this may be a
serious deficiency. E.g. Ant 1.6 introduces
support for namespaces in various forms; in order
to support completing elements in a
namespace-aware fashion, it may be necessary to
have the XML module grok them. For example, if you
have a script

<project default="x">
    <typedef file="mylib.xml" uri="urn:foo"/>
    <target name="x">
        <!-- CURSOR HERE -->
    </target>
</project>

and mylib.xml contains

<antlib>
    <taskdef name="foo" classname="Foo"/>
</antlib>

then an appropriate completion would be

    <f:foo xmlns:f="urn:foo"/>

or

    <foo xmlns="urn:foo"/>

or just

    <f:foo/>

if xmlns:f="urn:foo" is added to <project>.

It would be possible - with a fair amount of extra
work - to implement the first or second
possibilities in the GrammarQuery provider, rather
than the xml/text-edit module, if this clause from
the GrammarResult Javadoc

"It can have children representing mandatory
content. However it is up to client if it uses the
mandatory content."

guaranteed anything: you could manually examine
the document tree looking for existing namespace
prefixes, and if necessary add one on the element
itself. Unfortunately it seems that
ElementResultItem in fact ignores any content and
pays attention only to the element name:

public ElementResultItem(GrammarResult res) {
    super(res.getNodeName());
    foreground = Color.blue;
    startElement = true;
}

So unless I am missing something, it is impossible
in general to provide correct namespace-aware XML
completion with the current API implementation. I
guess you could try to return an Element
GrammarResult with a "name" like

   "f:foo xmlns:f=\"urn:foo\""

But this is an awful hack, and it will not work if
xml/text-edit attempts to insert a close tag.

Note: XSLGrammarQuery does provide completion on
elements in the namespace
http://www.w3.org/1999/XSL/Transform with whatever
namespace prefix. However it needs to manually
interpret prefixes and xmlns attributes. (See
updateProperties().) And this trick will only work
if the document already has a defined prefix for
the element to be completed - not a problem for
XSLGQ since this will hold for stylesheets, but a
problem for Ant scripts where it cannot be assumed
that the user has already defined namespace prefixes.