Set div width equal to window size

With a fixed width: 1300px;, you can not have a responsive to the window size div

As codehorse said, add

.container {
   width: 100%;
}

but add

html, body {
    height:100%;
    width:100%;
    margin:0;
    padding:0;
}

to you css as width: 100%; will render correctly if the dimension of the parent elements have been set beforehand!!

working demo

change container to

.container {
   width: 100%;
}

If you have the div width in px format it wont be responsive. It has to be in % format. If you put the inner content as width:100% The inner content will have a width of the '.container'.
So in order to get the inner div to be the window width you should have the .container to be equal to the window width.

CSS:

.container{
   width:100%;
}

Or if you want we can use jQuery:

 var w = $(window).width();
 $('.content').css('width', w);

It looks like you want to open your content div as a modal window, hiding rest of the body elements. Try this JSBIN DEMO

.content {
  position: fixed;
  top: 0;
  left: 0;
  bottom:0;
  right:0;
}