Use conditional statements at the top of the HTML document:
<!--[if lt IE 7 ]> <html lang="en" class="no-js ie6"> <![endif]-->
<!--[if IE 7 ]> <html lang="en" class="no-js ie7"> <![endif]-->
<!--[if IE 8 ]> <html lang="en" class="no-js ie8"> <![endif]-->
<!--[if IE 9 ]> <html lang="en" class="no-js ie9"> <![endif]-->
<!--[if (gt IE 9)|!(IE)]><!--> <html lang="en" class="no-js"> <!--<![endif]-->
Then in the CSS add
.ie7 button { font-size:0;display:block;line-height:0 }
Taken from HTML5 Boilerplate - more specifically paulirish.com/2008/conditional-stylesheets-vs-css-hacks-answer-neither/