Run this in the dev tools console to ensure none of your stylesheets have too many rules / selectors for legacy IE (IE9 and below).

countCSSRules.js

function countCSSRules() {
    var results = '',
        log = '',
        slen = document.styleSheets.length;
    if (!document.styleSheets) {
        return;
    }
    for (var i = 0; i < slen; i++) {
        countSheet(document.styleSheets[i]);
    }
    function countSheet(sheet) {
        var count = 0;

        // Added a workaround for Firefox Security Error when accessing cross-domain 
        // stylesheets. See http://stackoverflow.com/a/23613052/13019

        // cssRules respects same-origin policy, as per
        // https://code.google.com/p/chromium/issues/detail?id=49001#c10.
        try {
            // In Chrome, if stylesheet originates from a different domain,
            // sheet.cssRules simply won't exist. I believe the same is true for IE, but
            // I haven't tested it.
            //
            // In Firefox, if stylesheet originates from a different domain, trying
            // to access sheet.cssRules will throw a SecurityError. Hence, we must use
            // try/catch to detect this condition in Firefox.
            if (sheet && sheet.cssRules) {
                for (var j = 0, l = sheet.cssRules.length; j < l; j++) {
                    if (!sheet.cssRules[j].selectorText) {
                        if (sheet.cssRules[j].cssRules) {
                            for (var m = 0, n = sheet.cssRules[j].cssRules.length; m < n; m++) {
                                if(sheet.cssRules[j].cssRules[m].selectorText) {
                                    count += sheet.cssRules[j].cssRules[m].selectorText.split(',').length;
                                }
                            }
                        }
                    }
                    else {
                        count += sheet.cssRules[j].selectorText.split(',').length;
                    }
                }
     
                log += '\nFile: ' + (sheet.href ? sheet.href : 'inline <style> tag');
                log += '\nRules: ' + sheet.cssRules.length;
                log += '\nSelectors: ' + count;
                log += '\n--------------------------';
                if (count >= 4096) {
                    results += '\n********************************\nWARNING:\n There are ' + count + ' CSS rules in the stylesheet ' + sheet.href + ' - IE will ignore the last ' + (count - 4096) + ' rules!\n';
                }
            }
        } catch(e) {
            // Rethrow exception if it's not a SecurityError. Note that SecurityError
            // exception is specific to Firefox.
            if(e.name !== 'SecurityError') {
                throw e;
                return;
            }
        }
        return results;
    }
    console.log(log);
    console.log(results);
};
countCSSRules();