<!--[if lt IE 7]>
<p class="browserwarning">You are using an <strong>outdated</strong> browser. Please <a href="http://browsehappy.com/">upgrade your browser</a> to improve your experience.</p>
<![endif]-->
<!--[if lt IE 8]>
<p class="browserupgrade">You are using an <strong>outdated</strong> browser. Please <a href="http://outdatedbrowser.com/en">upgrade your browser</a> to improve your experience.</p>
<![endif]-->
Best,
Karl-Heinrich
P.S.: Or make it easy on your self and grab the script here. You can style the message anyway you want.