The default 404.html page returns the 200 server response. Often need get the 404 server answer.
The solution is very simple in SPIP.
1. Create 404.html in squelettes/ folder
2. Put the following php code to the file
header('HTTP/1.0 404 Not Found');
echo "<h1>404 Not Found</h1>";
echo "The page that you have requested could not be found.";
3. Check the response