7.align-elements-using-the-justify-content-property.html

<style>
  #box-container {
    background: gray;
    display: flex;
    height: 500px;
    justify-content: center;
  }
  #box-1 {
    background-color: dodgerblue;
    width: 25%;
    height: 100%;
  }

  #box-2 {
    background-color: orangered;
    width: 25%;
    height: 100%;
  }
</style>

<div id="box-container">
  <div id="box-1"></div>
  <div id="box-2"></div>
</div>