top level xsl:stylesheet tag mark in red

This is the content of my test file.

<?xml version="1.0"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">
</xsl:stylesheet>

xsl:stylesheet is in red in the opening tag. I do not understand why. Since I am not a XML expert I would like to make sure before I file a bug. The transformation goes without problem anyway.

Jacques