Edit in JSFiddle

Result
HTML
CSS

<body>
  <div class="contain">
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
  </div>
</body>

        html,
        body {
          margin: 0;
          padding: 0;
          width: 100%;
          height: 100%;
        }
        
        .contain {
          width: 100%;
          height: 100%;
        }
        
        .item {
          float: left;
          background-color: darkslategrey;
          border: 1px solid black;
          margin: 5%;
          width: 20%;
          height: 0;
          padding-bottom: 20%;
        }