How can I make a div take the full width of the page when it is inside another div that have 90% width

Answer 1

You can position the child relative and left: 50% and then translate it by -50% in the x-axis to re-align it with the edge of the screen. This works because left: 50% is half of the parent width while the transform: translateX(-50%) is half of the element itself. This relies on the original container being centered so may not work for all cases.

.container {
  background: gray;
  width: 80%;
  margin: auto;
}

.child {
  position: relative;
  left: 50%;
  transform: translateX(-50%);
  width: 100vw;
  background: red;
}

<div class='container'>
  <div>Centered</div>
  <div class='child'>Something</div>
  <div>Centered</div>
  <div>Centered</div>
  <div>Centered</div>
</div>

Answer 2

Consider negative margin.

.container {
  width: 90%;
  margin: 0 auto;
  padding: 10px 0;
  background: red;
}

.child {
  height: 50px;
  background: blue;
  margin: 0 calc(-5% / 0.9); 
  /* 
    The container is taking 90% of 100% so we are missing 10%
    we plit this value on both sides (5% + 5%) and since percentage is relative
    to parent so to make the percentage inside .child relative to body
    we divide by the percentage of container
    
    it should be then 5%/0.9 or 5%*1.11
  
  */
}

body {
  margin: 0;
}

<div class='container'>
  <div class='child'>
    something
  </div>
</div>

With CSS variable you can have something more dynamic:

.container {
  width: calc(var(--s)*100%);
  margin: 5px auto;
  padding: 10px 0;
  background: red;
}

.child {
  height: 50px;
  background: blue;
  margin: 0 calc((-50% * (1 - var(--s))) / var(--s)); 
}

body {
  margin: 0;
}

<div class='container' style="--s:0.8">
  <div class='child'>
    something
  </div>
</div>

<div class='container' style="--s:0.5">
  <div class='child'>
    something
  </div>
</div>
<div class='container' style="--s:0.2">
  <div class='child'>
    something
  </div>
</div>

In case the container is not centered simply put all the missing margin on one side:

.container {
  width: calc(var(--s)*100%);
  margin: 5px 0;
  padding: 10px 0;
  background: red;
}

.child {
  height: 50px;
  background: blue;
  margin-right:calc((-100% * (1 - var(--s))) / var(--s)); 
}

body {
  margin: 0;
}

<div class='container' style="--s:0.8">
  <div class='child'>
    something
  </div>
</div>

<div class='container' style="--s:0.5">
  <div class='child'>
    something
  </div>
</div>
<div class='container' style="--s:0.2">
  <div class='child'>
    something
  </div>
</div>

PS: the use of vw isn't a good idea because it includes the width of the scroll. So you will have overflow.

.box {
  height:50px;
  background:red;
  width:100vw;
  border:5px solid green;
  box-sizing:border-box;
}

body {
 margin:0;
 min-height:200vh;
}

<div class="box"></div>

Answer 3

.child { 
   width: 100%; 
   position: absolute;
}