ie6 =         (parseFloat(navigator.appVersion.split("MSIE")[1]) != 6) ? false : true;
ie7 =         (parseFloat(navigator.appVersion.split("MSIE")[1]) != 7) ? false : true;
ie8 =         (parseFloat(navigator.appVersion.split("MSIE")[1]) != 8) ? false : true;
firefox =     (navigator.userAgent.indexOf("Firefox")==-1) ? false : true;
opera =     (navigator.userAgent.indexOf("Opera")  ==-1) ? false : true;

function loadCSS () {
    link = document.getElementById('cssLink');
    if (ie6) {link.href = 'css/shopIE6.css'}    
    if (ie7) {link.href = 'css/shopIE7.css'} 
    if (ie8) {link.href = 'css/shopIE8.css'} 
    if (opera) {link.href = 'shopOpera.css'}
}

loadCSS();