<header class="clearfix">
    <ul class="header-left">
      <li><span>L1</span></li>
      <li><span>L2</span></li>
    </ul>
    <ul class="header-right">
      <li><span>R1</span></li>
      <li><span>R2</span></li>
    </ul>
  </header>
ul {
  padding: 0;
}
li {
  display: inline-block;
  margin: 5px; 
  padding: 5px 5px;
  border: solid 1px #000000;
}
header {
  background-color: red;
}
.header-left {
  background-color: pink;
  float: left;
}
.header-right {
  background-color: yellow;
  float: right;
}
.clearfix:after {
  display: block;
  content: "";
  clear: both;
}

External CSS

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External JavaScript

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