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: 20vw;
            height: 20vw;
        }