how to put container in center of page

IMG.displayed {
    display: block;
    margin-left: auto;
    margin-right: auto }
...
<IMG class="displayed" src="..." alt="...">

div.InSkinAlignCenter {
margin: 0 auto;
}

P { text-align: center }
H2 { text-align: center }

#container
{
    width: 400px;
    margin-left: auto;
    margin-right: auto;
}