XML is not backward compatible?

[Guennadiy VANIN]

I use XSLT processor (win32 executable, from 1999 (it is from the book of my grandma), that requires from my.xsl file
<xsl:stylesheet xmlns:xsl="http://www.w3.org/XSL/Transform/1.0"
version="1.0">
while my browser MS IE6.0 requires
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
otherwise they resp. give error
I thought that it is well established practice, almost a law, to make standards/applications backward compatible (if not forward)
Any comments
I thank all buds in other threads who responded, my time on Internet is gone