The IE6 and Netscape 6 all use the new XSL namespace
The correct way to declare an XSL style sheet according to the W3C XSL Recommendation is:

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

<xsl:transform version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
Note: The xmlns:xsl="http://www.w3.org/1999/XSL/Transform" identifies the official W3C XSL recommendation namespace. If you use this namespace, you must also include the attribute version="1.0".

But if you want to support IE5 you should use the old declaration (that is already OUTDATED)

<xsl:stylesheet
xmlns:xsl="http://www.w3.org/TR/WD-xsl">