JSFiddle - React, Tailwind, and code Playground

HTML

<div id ="cont">
    <div class ="box" > box1 </div>
    <div class ="box" > box2 </div>
    <div class ="box" > box3 </div>
</div>
<div id="foo">footer</div>

CSS

#cont {
  width:950px;
  border:1px solid red;
}
#cont:after {
    content: '';
    display: block;
    clear: both;
}
.box {
  width:300px;
  height:100px;
  border:1px solid black;
  background-color:orange;
  float:left;       
}
#foo {
  width:100px;
  background-color:blue;
  clear:both;
}