This is a nice way of asking the browser to give you focus of a form field. Better yet, it's simple. If no other field has focus, you can have it. If some other field has focus, leave it there.

<form action="" method="post">
    <fieldset id="your_login_form" class="">
        <legend>Your login form</legend>
        
        <label for="username">Username</label><input type="text" name="username" value="" id="username">
        <br>
        <label for="password">Password</label><input type="password" name="password" value="" id="password">
    </fieldset>
    
    <p>
        <button type="submit">Login</button>
    </p>
</form>

<script>
/* <![CDATA[ */
    $('#username').politeFocus();
/* ]]> */
</script>