Building on jetli13's answer 10 years later with no IE selector limits to be seen but styledcomponents and mega CSS in JS generator's in the future, I added the following
- Warning over 1000 styles
 
- Script works even if you have 3rd party external rules
 
- Shows addition of styles per 
style tag or external stylesheets 
- Lets you expand individual CSS Ruleset's
 
Script - Copy and run in console
You can run it after a route change in a SPA application to see how many rules got added.
var
  styleSheets = document.styleSheets,
  totalStyleSheets = styleSheets.length,
  overAllDocumentRules =0;
try {
for (var j = 0; j < totalStyleSheets; j++){
    try{
  var
    styleSheet = styleSheets[j],
    rules = styleSheet.cssRules,
    totalRulesInStylesheet = rules.length,
    totalSelectorsInStylesheet = 0;
  for (var i = 0; i < totalRulesInStylesheet; i++) {
    if (rules[i].selectorText){
      totalSelectorsInStylesheet += rules[i].selectorText.split(',').length;
    }
  }
}
catch(err){
    console.warn("skipping>", styleSheet.href, err);
    console.warn(styleSheet)
}
if(totalRulesInStylesheet>1000) console.error("This stylesheet has over 1000 rules")
  console.log("Stylesheet: "+styleSheet.href);
  console.log(styleSheet)
  console.info("Total rules: "+totalRulesInStylesheet);
  overAllDocumentRules += totalRulesInStylesheet;
  console.warn("Total Document Rules > "+ overAllDocumentRules)
}
}
catch(err){
    console.warn("Error",err)
}
Cross Domain Stylesheet Rules a.k.a What is your 3rd party doing
If you need 3rd party styles in Safari web inspector do this:

Output
