Edit in JSFiddle

document.addEvent('domready', function(){
  $$('input, textarea').addEvent('focus', function(){
    $$('input, textarea').removeClass('focused');
    $(this).addClass('focused');
    
  })
});

<form id="commentform" action="" method="get">
    <p>
        <label for="name">Name : </label>
        <input id="name" name="name" tabindex="1" type="text">
    </p>
    <p>
        <label for="email">Email : </label>
        <input id="email" name="email" tabindex="2"  type="text">
    </p>
    <p>
        <label for="url">Website :</label>
        <input id="url" name="url" tabindex="3"  type="text">
    </p>
    <p>
      <label for="comment">Comment :</label>
        <textarea id="comment" name="comment" cols="" rows="" tabindex="4"></textarea>
    </p>
    <p align="center">
        <input name="submit" type="button" tabindex="5" value="Submit">
    </p>
</form>

<!--
body{
    font-family: Verdana, Geneva, sans-serif;
    font-size: 12px;
}
#commentform{
    border: 1px solid #cac9b9;
    padding: 5px;
    width: 450px;
}
#commentform p {
margin-bottom: 5px;    
}
#commentform label {
    float: left;
    width: 80px;
}
#commentform input,#commentform textarea {
border: 1px solid #cac9b9; background: #fff;
}
#commentform textarea{
    width: 200px;
}
#commentform .focused {
    border:1px solid #888;
    background-color: #ffffaa;
    /*css3 outline*/
    outline: 2px solid #ffffaa; 
}
-->