Escaping a CSS column with a fixed width - html

Question

If you’re trapped in a page wrap - say all the contents of the page are in a div whose width is 900px, then you want one div WITHIN that that’s the full page width. What’s the easiest way to do this?
I know you can end the 900px div, do the full width div, and then start another 900px div, but is there a way to style the inner div so you don't have to escape it? 100vw works for making it the right size but doesn't position it in the right spot.
So simplistic example:
<div style="width:900px;margin-right:auto;margin-left:auto;display:block;">
<p>text text</p>
<div style="width:100vw;">
<p>I want this section to be the full page width and centered</p>
</div>
<p>text text</p>
</div>
Thanks!

Answer 1

You can use negative left margin (-50vw + half parent width).
body {margin: 0;}
#a {background: red;}
#b {background: green; margin-left: calc(-50vw + 200px)
<div id="a" style="width:400px;margin-right:auto;margin-left:auto;display:block;">
<p>text text</p>
<div style="width:100vw;" id="b">
<p>I want this section to be the full page width and centered</p>
</div>
<p>text text</p>
</div>
For this code example I've added IDs (for cleaner CSS styles) and change parent div to 400px (because there is smaller window).

Answer 2

I don't recommend trying to make a child div "escape" its parent because going with that approach will require pointlessly complicated CSS. You can accomplish what you want with a container div and a couple nested children which is a much simpler solution:
.narrow {
width: 300px;
margin: 0 auto;
background-color: tomato;
padding: 16px;
}
.full {
background-color: gold;
padding: 16px;
}
<div>
<div class="narrow">
<p>text text text</p>
</div>
<div class="full">
<p>more text or image or whatever</p>
</div>
<div class="narrow">
<p>text text text</p>
</div>
</div>

Answer 3

I would argue that the way you are trying to solve the issue is not very helpful for an actual website. Normally, you would have a container, your top div, which contains its lower elements. Making a child element go outside its parent div like you seem to want goes against that mentality.
Of course, sometimes you may want to put an element outside its parent, and you can use pavel's answer. For example, maybe you want to animate a line moving. You would then offset that element by -100% and then change that offset to give it the impression of movement. But that would be a special case.
To solve your problem, I would use the following structure:
Here is a link to the example too.
<div class='container'>
<div class='thin'>
<p>text text</p>
</div>
<div class='full-width'>
<p>I want this section to be the full page width and centered</p>
</div>
<div class='thin'>
<p>text text</p>
</div>
</div>
.container{
text-align:center;
padding: 0 5vw; //padding of 5vw to the left and right
}
.thin{
width:80vw;
margin: 0 auto;
background-color:yellow;
}
.full-width{
background-color:green;
}