100% width for child element

Question

<div class="container" style="width: 900px; margin: 0 auto;">
   <div class="width-full">
     <!-- Full width container goes here -->
   </div>   
</div>

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