Count the number of rules and selectors for CSS files on the page. Flags up the >4096 threshold that confuses IE. — This snippet has been modified to count more than just the first level of CSSStyleRule objects within CSSMediaRule.

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++) {
        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';
      }
    }
  }
  console.log(log);
  console.log(results);
};
countCSSRules();

countCSSRulesWithImport.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++) {
        var rule = sheet.cssRules[j];
        if (rule instanceof CSSImportRule) {
          countSheet(rule.styleSheet);
        }
        if( !rule.selectorText ) {
          continue;
        }
        count += rule.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();