Selector for one tag directly followed by another tag

Do you mean to style A given that it has a B element directly inside or followed? Like this:

<A>
    <B>
    </B>
</A>

// OR

<A>
</A>
<B>
</B>

You can't do such a thing in CSS (yet). Eric Meyer states that this kind of selector has been discussed quite a few times on the CSS mailing list, and isn’t doable. Dave Hyatt, one of the core WebKit developers, comments with a good explanation of why it can’t be done.

Check out: Shaun Inman's blog post and the comment by Eric Meyer.
David Hyatt weighs in, too.

You can’t in css.

Edit: To be a bit more helpful, if you use for example jQuery (a JavaScript library), you can use .prev().