index.html

<!DOCTYPE html>
<html lang="en" dir="ltr">
  <head>
    <meta charset="utf-8">
    <title>CSS Flexbox</title>
    <link rel="stylesheet" href="./index.css">
  </head>
  <body>
    <div class="blue"></div>
    <div class="red"></div>
    <div class="yellow"></div>
    <div class="blue"></div>
    <div class="red"></div>
    <div class="yellow"></div>
    <div class="blue"></div>
    <div class="red"></div>
    <div class="yellow"></div>
  </body>
</html>