You can use the following XSLT: new template that matches "node() | @*" copies the the nodes and attributes as it is. And instead of copy-of I've used apply-templates:
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns="http://www.w3.org/1999/xhtml">
<xsl:output method="xml"
version="1.0"
encoding="UTF-8"
doctype-system="http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"
doctype-public="-//W3C//DTD XHTML 1.1//EN"
indent="yes"/>
<xsl:strip-space elements="*"/>
<xsl:param name="base"
select="/content/@base-url"/>
<!-- PAGE TEMPLATE -->
<xsl:template match="/content">
<html lang="en">
<head>
...
</head>
<body>
<div class="content">
<xsl:apply-templates select="article"/>
</div>
</body>
</html>
</xsl:template>
<xsl:template match="a">
<a href="{concat($base, @href)}">
<xsl:value-of select="."/>
</a>
</xsl:template>
<!-- ARTICLE NODE TEMPLATE -->
<xsl:template match="article">
<h2>
<xsl:value-of select="@title"/>
</h2>
<h3>
<xsl:value-of select="@timestamp"/>
</h3>
<xsl:apply-templates/>
</xsl:template>
<xsl:template match="node() | @*">
<xsl:copy>
<xsl:apply-templates select="node() | @*"/>
</xsl:copy>
</xsl:template>
</xsl:stylesheet>