How can I expand a child div to 100% screen width if the container div is smaller?

The parent element of the whole page is a centered div limited to a max-width of 960px. All other elements on the page are children of that parent div. The simplified structure is the following:

<div id="parent">
  <div id="something"></div>
  <div id="wide-div"></div>
  <div id="something-else"></div>
</div>

While the parent div shouldn't expand beyond a width of 960px, the div I called "wide-div" here should fill the entire width of the screen. It contains a single image that is wider than the 960px, and it should set a different background color for the entire width of the screen.

I can't easily take that div out of the parent div, it would mess up other parts of my layout and it would make the whole thing rather awkward.

I found a few tricks on how you can achieve this, but none seemed to fit my requirements. My design is responsive, or at least I'm trying to achieve that. The tricks I found relied on knowing the size of the involved elements, which is not fixed in my case.

Is there a way to expand the inner div to the full screen width in a responsive layout?

You can set the width based on the vw (viewport width). You can use that value too using the calc function, to calculate a left-margin for the div. This way you can position it inside the flow, but still sticking out on the left and right side of the centered fixed-width div.

Support is pretty good. vw is supported by all major browsers, including IE9+. The same goes for calc(). If you need to support IE8 or Opera Mini, you're out of luck with this method.

-edit-

As mentioned in the comments, when the content of the page is higher than the screen, this will result in a horizontal scrollbar. You can suppress that scrollbar using body {overflow-x: hidden;}. It would be nice though to solve it in a different way, but a solution using left and rightlike presented in Width:100% without scrollbars doesn't work in this situation.

-edit 2021-

Another work-around for the scrollbars, which may be acceptable or not depending on your situation:
By making the green div a little bit smaller, say 20px, you can keep a bit of space for the scrollbar. Half that reserved width can be added to the margin, to keep the wide div centered:

#wide-div {
  width: calc(100vw - 20px);
  margin-left: calc(-50vw + 50% + 10px);

div {
  min-height: 40px;
  box-sizing: border-box;
}
#container {
  position: relative;
}
#parent {
  width: 400px;
  border: 1px solid black;
  margin: 0 auto;
}

#something {
  border: 2px solid red;
}

#wide-div {
  width: calc(100vw - 20px);
  margin-left: calc(-50vw + 50% + 10px);
  border: 2px solid green;
}

<div id="container">
<div id="parent">
  <div id="something">Red</div>
  <div id="wide-div">Green


<br>Green
<br>Green
<br>Green
<br>Green
<br>Green
<br>Green
<br>Green
</div>
  <div id="something-else">Other content, which is not behind Green as you can see.</div>
</div>
</div>

After much research, I found this solution: Creating full width (100% ) container inside fixed width container. I think that it is the best solution because it does not depend on any external factor, only the div that you want to expand.

<div class="container" style="width: 750px; margin: 0 auto;">
   <div class="row-full">
     --- Full width container ---
   </div>   
</div>

.row-full{
     width: 100vw;
     position: relative;
     margin-left: -50vw;
     left: 50%;
}

You can now do this

.full-width {
    margin-left: calc(50% - 50vw);
    margin-right: calc(50% - 50vw);
}

or this

.full-width {
    width: 100vw;
    position: relative;
    left: 50%;
    right: 50%;
    margin-left: -50vw;
    margin-right: -50vw;
}

More details: https://css-tricks.com/full-width-containers-limited-width-parents/

Typically the responsive element, bootstrap or Foundation, allow you to add a "row" element. You can put the "wide-div" outside an element with "row" and it should expand to take up the full width.

Alternatively, you can use absolute positioning for that element which ignores most inherited settings:

.wide-div {
    position: absolute;
    left: 0;
    right: 0;
}

I'm a little surprised no one offered the following in the last 4 years. The css position:fixed property pulls the item out and scales it in relation to the window. There are some cases where maybe this doesn't work, but if you're Javascripting a modal box or something, this works fine.

.wide-div{
	position:fixed;
	top:0px;
	left:0px;
	width:100%;
    height:100%; // in case you need to cover the height as well
	background-color:rgba(0,0,0,.8); //just so you can see the div is on top of your content
	z-index:1; // you may need to adjust the index of your other elements as well
}

You can use vw. Demo http://jsfiddle.net/fsLhm6pk/

.parent {
  width: 200px;
  height: 200px;
  background: red;
}

.child {
  width: 100vw;
  height: 50px;
  background: yellow;
}

<div class='parent'>
  <div class='child'></div>
</div>

You are right, this won't work with centered div. Try this instead:

EDIT http://jsfiddle.net/fsLhm6pk/1/

.parent {
  width: 200px;
  height: 200px;
  background: red;
  margin: 0 auto;
}

.child {
  width: 100%;
  left: 0;
  right: 0;
  position: absolute;
  height: 50px;
  background: yellow;
}

<div class='parent'>
  <div class='child'></div>
</div>

My 50cent here..

Although I find other people's answers correct too, i think it needs to be mentioned an old css trick to archive this

.wide-div {    
   padding: 0 9999%;
   margin: 0 -9999%;
}

Again in this case too, if a horizontal scrollbar appears you can fix this using

body {overflow-x: hidden;} 

Depending the case, the difference in this code is that the inner content here stays aligned with the parent width

body {overflow-x: hidden;} 

div {
  min-height: 40px;
  box-sizing: border-box;
}
#parent {
  width: 400px;
  border: 1px solid black;
  margin: 0 auto;
}

#something {
  border: 2px solid red;
}
#something-else {
  border: 2px solid red;
}

#wide-div {
  padding: 0 9999%;
	margin: 0 -9999%;
  border: 2px solid green;
}

<div id="parent">
  <div id="something">Red</div>
  <div id="wide-div">Wide</div>
  <div id="something-else">Other content</div>
</div>