if( typeof( window.innerWidth ) == 'number' ) {
    winw = window.innerWidth;
    winh = window.innerHeight;
} else if( document.documentElement && ( document.documentElement.clientWidth || document.documentElement.clientHeight ) ) {
    winw = document.documentElement.clientWidth;
    winh = document.documentElement.clientHeight;
} else if( document.body && ( document.body.clientWidth || document.body.clientHeight ) ) {
    winw = document.body.clientWidth;
    winh = document.body.clientHeight;
}

var page=document.getElementById("pageID")

page.style.left=(winw-800)/2-10+"px"
page.style.top=(winh-600)/2-10+"px"
page.style.width=800+"px"
page.style.height=600+"px"