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

Question

how can I make the child take the full width of the page

---

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

.container {
    width: 90%;
    margin: 0 auto;
 }

.child {}

Answer 1

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>