<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">

<head>

	<meta http-equiv="content-type" 

		content="text/html;charset=utf-8" />

	<link type="text/css" rel="stylesheet" href="stylesheet.css"/>

	<title>CSS formatierte Tabelle</title>

results in this Warning

DOCTYPE Override in effect!
The detected DOCTYPE Declaration "<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">" has been suppressed and the DOCTYPE for "XHTML 1.0 Strict" inserted instead, but even if no errors are shown below the document will not be Valid until you update it to reflect this new DOCTYPE.

What do i do wrong ?

Thank you for your help

frank
NOTE: Whenever possible, give the address of the document you were checking.