retaining "xml:" prefix on attribute name

Hello,

I'm trying to make an XSLT stylesheet include the following element in
its result document.

<grammar version="1.0" xml:lang="en-US" xmlns="http://www.w3.org/2001/06/grammar" root="root">

The resulting document must be html.
However, when I use <xsl:output method="html"/>, I get

<grammar version="1.0" lang="en-US" xmlns="http://www.w3.org/2001/06/grammar" root="root"> (i.e., the "xml:" prefix on the lang attribute is removed!).

When I use <xsl:output method="xml"/>, the "xml:" prefix remains.
I however need html, not xml, output.  I am using libxslt.
Is it possible to retain the "xml:" prefix on the lang attribute in HTML
output?

Thanks,
Saverio

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