Solution:
I assume you meant html, not css. To add a placeholder text you simply add: placeholder=”Your placeholder”.
In your form it would be:
<label for="user_login">Username<br>
<input type="text" name="log" id="user_login" placeholder="Your Username" class="input" value="" size="20">
</label>
Javascript solution:
jQuery('#user_login').attr( 'placeholder', 'Username' );
JQuery('#user_pass').attr( 'placeholder', 'Password' );
That should work, if it doesn’t try this from github https://gist.github.com/makeusabrew/985739