var viewportwidth;
var viewportheight;

// the more standards compliant browsers (mozilla/netscape/opera/IE7) use window.innerWidth and window.innerHeight

if (typeof window.innerWidth != 'undefined')
{
    viewportwidth = window.innerWidth,
    viewportheight = window.innerHeight
}

// IE6 in standards compliant mode (i.e. with a valid doctype as the first line in the document)

else if (typeof document.documentElement != 'undefined'
   && typeof document.documentElement.clientWidth !=
   'undefined' && document.documentElement.clientWidth != 0)
{
     viewportwidth = document.documentElement.clientWidth,
     viewportheight = document.documentElement.clientHeight
}

// older versions of IE

else
{
     viewportwidth = document.getElementsByTagName('body')[0].clientWidth,
     viewportheight = document.getElementsByTagName('body')[0].clientHeight
}