Declare all namespaces on root element - xslt

Question

I would like a transform such that all the namespaces in a document are declared on the documents root element. So that
<a>
<ns1:b xmlns:ns1="urn:ns1"/>
<ns2:c xmlns:ns2="urn:ns2"/>
</a>
becomes
<a xmlns:ns1="urn:ns1" xmlns:ns2="urn:ns2">
<ns1:b/>
<ns2:c/>
</a>
It doesn't matter if the namespaces appear on the local declarations the point is that they should all have global scope.

Answer 1

Not sure why the placement of the namespace declarations should make a difference. Still, if you want, you could try something like:
XSLT 1.0
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml" version="1.0" encoding="UTF-8" indent="yes"/>
<xsl:strip-space elements="*"/>
<!-- identity transform -->
<xsl:template match="#*|node()">
<xsl:copy>
<xsl:apply-templates select="#*|node()"/>
</xsl:copy>
</xsl:template>
<xsl:template match="/a">
<a xmlns:ns1="urn:ns1" xmlns:ns2="urn:ns2">
<xsl:apply-templates/>
</a>
</xsl:template>
</xsl:stylesheet>
Or in a more generic way:
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml" version="1.0" encoding="UTF-8" indent="yes"/>
<xsl:strip-space elements="*"/>
<!-- identity transform -->
<xsl:template match="#*|node()">
<xsl:copy>
<xsl:apply-templates select="#*|node()"/>
</xsl:copy>
</xsl:template>
<xsl:template match="/*">
<xsl:copy>
<xsl:copy-of select="//namespace::*"/>
<xsl:apply-templates select="#*|node()"/>
</xsl:copy>
</xsl:template>
</xsl:stylesheet>
These may work or not, depending on the whim of your XSLT processor.

Answer 2

There are documents for which this is impossible, for example:
<a>
<ns1:b xmlns:ns1="urn:ns1"/>
<ns1:b xmlns:ns1="urn:ns2"/>
</a>
and you need to decide what you want done about these.
The "generic" solution from #michael.hor257k should generally work under XSLT 2.0+, with caveats:
(a) it will give you an error if there are conflicting namespace declarations, as described
(b) although this syntax is also accepted under XSLT 1.0, the output isn't well-defined under XSLT 1.0, because serializers have considerable freedom over where to put the serialized namespace declarations.