Prova questo codice che a me funziona benissimo.

Codice PHP:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html>
    <head>
        <title>prova</title>
        <meta http-equiv="content-type" content="text/html; charset=iso-8859-1">
        <meta name="generator" content="HAPedit 3.1">
        <style type="text/css">
            div#nifty{ margin: 0 10%;background: #9BD1FA}
            b.rtop,b.rbottom{display:block;background:#FFF}
            b.rtop b,b.rbottom b{display:block;height:1px;overflow:hidden;background:#9BD1FA}
            b.r1{margin:0 5px}
            b.r2{margin:0 3px}
            b.r3{margin:0 2px}
            b.rtop b.r4,b.rbottom b.r4{margin:0 1px;height: 2px}
        </style>
    </head>
    <body>
        <div id="nifty">
            <b class="rtop"><b class="r1">[/b]<b class="r2">[/b]<b class="r3">[/b]<b class="r4">[/b][/b]
            prova
            <b class="rbottom"><b class="r4">[/b]<b class="r3">[/b]<b class="r2">[/b]<b class="r1">[/b][/b]
        </div>
    </body>
</html>