you don't want to copy the document, you want to change it, so don't do
this

                <xsl:copy-of select="document($full-path)/html:html/
html:body/*" namespace="http://www.w3.org/1999/xhtml"/>

acually that's a syntax error (didn't your processor complain?)
you meant

                <xsl:copy-of select="document($full-path)/html:html/
html:body/*"/>

but as I say you don't want a copy you want

<xsl:apply-templates select="document($full-path)/ html:html/
html:body/*"/>

together with the template yu posted t zap shape and an identity
template for the rest

<xsl:template match="@*|*">
<xsl:copy>
<xsl:apply-templates select="@*|node()"/>
</xsl:copy>
</xsl:template>

Alternatively you can use a catalog (or perhaps a command line switch
for your parser) or modify your input documents, so that they do not
reference  the XHTMl DTD

(for it is this, not XSLT that is adding the attributes).

David