!Important Declaration And Use Cases

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <title>CSS</title>
    <link rel="stylesheet" href="css/master.css" />
  </head>
  <body>
    <div class="one" style="background-color: red;">One</div>
  </body>
</html>

.one {
  background-color: #EEE !important;
}