A method that tries to remove CSS rules that do not apply to anything in the current page, aids in minimising test cases

minicss.js

(function minimizeCSS(){
  var s = document.styleSheets[0].cssRules;
  var j=s.length-1, ar=[]; 
  try{
    for(;j>=0;j--){
      if(document.querySelectorAll(s[j].selectorText).length>0)ar.push(s[j].cssText);
    }
  }catch(e){
    console.log('error for style #'+j)
  }
  document.body.appendChild(document.createElement('pre')).appendChild(document.createTextNode(ar.join('\n')));
})();