XHTML 1.0 was designed to be backwards compatible. Like MCookie said, the only hangup is that NS4 will not render <br/> (which is valid), but will render <br /> (which is also valid).
Internet Explorer has a couple hangups on an XHTML 1.1 document, with the doctype, that is.
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"The first step to confirming there is a bug in someone else's work is confirming there are no bugs in your own."
June 30, 2001
author, Verbosio prototype XML Editor
author, JavaScript Developer's Dictionary https://alexvincent.us/blog