<div class="container">
   <ul class="feed">
     <li><div class="card">Card 1</div></li>
     <li><div class="card">Card 2</div></li>
     <li><div class="card">Card 3</div></li>
     <li><div class="card">Card 4</div></li>
     <li><div class="card">Card 5</div></li>
     <li class="nested">
       <ul>
         <li><div class="card">Card A</div></li>
         <li><div class="card">Card B</div></li>
         <li><div class="card">Card C</div></li>
       </ul>
     </li>
     <li><div class="card">Card 6</div></li>
     <li><div class="card">Card 7</div></li>
     <li><div class="card">Card 8</div></li>
     <li><div class="card">Card 9</div></li>
     <li><div class="card">Card 10</div></li>
   </ul>
</div>
@import url('https://fonts.googleapis.com/css?family=Roboto:300,400');

* { 
  box-sizing: border-box; 
}

body {
    font-family: Arial, Helvetica, sans-serif;
    font-size: 14px;
    margin: 0;
	  background-color: grey;
}

ul {
  margin: 0;
  padding: 0;
  list-style: none outside none;
}

.container {
	max-width: 800px;
	margin: 0 auto;
}

.card {
	background-color: #fff;
	padding: 10px;
	margin: 10px;
	min-height: 300px;
}

.feed {
  display: flex;
  flex-wrap: wrap;
}

@supports (display: flex) and (display: contents) {
  .feed ul,
  .feed li {
    display: contents;
  }

  .card {
    flex: 1 0 40%;
  }
}

@supports (display: flex) and (not (display: contents)) {
  .feed li {
    flex: 1 0 50%;
  }

  .feed li.nested {
    flex-basis: 100%;
  }

  .feed li.nested ul {
    display: flex;
    flex-wrap: wrap;
  }
}

External CSS

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External JavaScript

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