countCSSRules.js

function countCSSRules() {
    var results = '',
        log = '';
    if (!document.styleSheets) {
        return;
    }
    for (var i = 0; i < document.styleSheets.length; i++) {
        countSheet(document.styleSheets[i]);
    }
    function countSheet(sheet) {
        var count = 0;
        if (sheet && sheet.cssRules) {
            for (var j = 0, l = sheet.cssRules.length; j < l; j++) {
                //Check for imports
                if (sheet.cssRules[j] instanceof CSSImportRule)
                {
                    countSheet(sheet.cssRules[j].styleSheet);
                }
                if( !sheet.cssRules[j].selectorText ) {
                    continue;
                }
                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';
            }
        }
    }
    console.log(log);
    console.log(results);
};
countCSSRules(); 

Works great, thanks! I made some suggested changes based on IE9 vs IE10+ selector limits, these are the changes:

if (count >= 65534) {
    results += '\n********************************\nWARNING:\n There are ' + count + ' CSS rules in the stylesheet ' + sheet.href + ' - IE10 and newer will ignore the last ' + (count - 4096) + ' rules!\n';
}
if (count >= 4096) {
    results += '\n********************************\nWARNING:\n There are ' + count + ' CSS rules in the stylesheet ' + sheet.href + ' - IE9 and older will ignore the last ' + (count - 4096) + ' rules!\n';
    results += '\n********************************\n@see https://blogs.msdn.microsoft.com/ieinternals/2011/05/14/stylesheet-limits-in-internet-explorer/';
}

https://gist.github.com/ElijahLynn/07ab9a7525b1511615db558473e52611