I think the best way would be this:
From HTML5 boilerplate:
<!--[if lt IE 7]> <html lang="en-us" class="no-js ie6 oldie"> <![endif]-->
<!--[if IE 7]> <html lang="en-us" class="no-js ie7 oldie"> <![endif]-->
<!--[if IE 8]> <html lang="en-us" class="no-js ie8 oldie"> <![endif]-->
<!--[if gt IE 8]><!--> <html lang="en-us" class="no-js"> <!--<![endif]-->
in JS:
if( $("html").hasClass("ie8") ) { /* do your things */ };
especially since $.browser
has been removed from jQuery 1.9+.