CSS – Make divs align horizontally

#container {
  background-color: red;
  overflow: hidden;
  width: 200px;
}

#inner {
  overflow: hidden;
  width: 2000px;
}

.child {
  float: left;
  background-color: blue;
  width: 50px;
  height: 50px;
}

<div id="container">
  <div id="inner">
    <div class="child"></div>
    <div class="child"></div>
    <div class="child"></div>
  </div>
</div>