SOURCE

<div class="box" >

  <div class="float">I am a float!</div>
  <p>I am content inside the container.</p>

</div>

.box{
	background-color: pink;
	border: 10px solid blue;
	/* display: flow-root; */
}

.float{
	float: left;
	border: 1px solid red;
	width: 100px;
	height: 100px;
	background-color: rgba(0, 0, 0, 0.3);
}