<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <meta http-equiv="X-UA-Compatible" content="ie=edge" />
    <title>Flex-order</title>
  </head>
  <body>
    <div>
      <ul class="flex-box">
        <li>Home</li>
        <li>About</li>
        <li>Portfolio</li>
        <li>Contact</li>
      </ul>
    </div>
  </body>
</html>

.flex-box {
        display: flex;
        flex-direction: row;
        justify-content: flex-start;
        align-items: center;
      }
      .flex-box li {
        padding: 10px;
        list-style-type: none;
        background: red;
        color: #fff;
        border: 1px solid tomato;
        box-shadow: 1px 0px 0px 1px tomato;
      }
      .flex-box :first-child {
        order: 1;
      }
      .flex-box :last-child {
        order: -1;
      }

/*Notes: Highest ordered items are displayed later than that of lowest order items*/

/*In the above example "Home" will be displayed last than other 3 and "Contact" will be displayed first as it's order is lower than others  */

/*Flex items with same order will be displayed in same order as source order such as one after other*/

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