function changeviewport(){
 var viewportwidth;

       viewportwidth = document.body.clientWidth;

if(viewportwidth<=910){
document.getElementById('container').style.left = 50 +'px';
} 
if(viewportwidth>910 && viewportwidth<1200){
       viewportwidthbrowser = document.body.clientWidth;

newleft = (viewportwidthbrowser/2)-410;
newleft2 = newleft;

document.getElementById('container').style.left = newleft2 +'px';
}
if(viewportwidth>=1200){
       viewportwidthbrowser = document.body.clientWidth;

newleft = (viewportwidthbrowser/2)-300;
newleft2 = newleft-210;

document.getElementById('container').style.left = newleft2 +'px';
}
}

changeviewport();