Thanks for the clear response.
The thing is I must be dumb because I can’t delete that white margin above the header, even after having modified the padding of #container to O.
Can I fill it with a negative number ? O.o
I don’t know if that’s useful but I’ve noticed also that I can change the page messing with that form :
body {
margin:0px;
padding:0px 0px;
font-family:Arial, Helvetica, sans-serif;
font-size:81%;
line-height:1.2em;
text-align:center;
}