<!DOCTYPE html>
<html>

  <head>
    <link rel="stylesheet" href="style.css">
    <script src="script.js"></script>
  </head>

  <body>
    <form action="DoNothing.aspx">
      <label for="txtFirstName" class="required">First Name</label>
      <br />
      <input id="txtFirstName" type="text" required />
    </form>
  </body>

</html>
/* Css Selector for tag of type label with class required */
label.required 
{
    font-weight: bold;
    color: orange;
}

/* Append text */
label.required::after
{
    content: "*"
}