Please explain validation error

From: Geier, Ronald - b.i.t.s. <Ro.Geier@bits-gmbh.de> Date: Wed, 11 Aug 2010 14:09:24 +0200 Message-ID: <40836C8770C2A440BABB55534687A5D40DD6BC16@VSMUCEXCH11.mysug.de> To: <www-validator-css@w3.org> · This archive was generated by hypermail 2.3.1 : Tuesday, 6 January 2015 21:40:47 UTC

Hello,

i am not able to vaildate the css on the page: www.buchbohne.de
xhtml check is passed, css check always stops with:

CSS 
Validator 
Error 
Target: http://www.buchbohne.de
Please, validate your XML document first!
Line 1
Column 3
The markup declarations contained or pointed to by the document type
declaration must be well-formed. 

What is wrong with the Declaration:

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
   "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="de" xml:lang="de">

Thank you

Ronald Geier