A solution would be to move the message into the BODY tag.
Placing the preview message just before the closing body tag prevents it from
interfering with the (x)HTML structure.
Reviewed-by: Georg Ringer
Reviewed-by: Oliver Klee
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter