for(var i = 0; i < sourceDocument.styleSheets.length; i++) {
    var styleSheet = sourceDocument.styleSheets[i];

    try {
      // In Firefox, if stylesheet originates from a different domain, trying
      // to access styleSheet.cssRules will throw a SecurityError. Hence, we must use
      // try/catch to detect this condition in Firefox.
      if(!styleSheet.cssRules) continue; // It's possible for rules to be undefined

      for(var j = 0; j < styleSheet.cssRules.length; j++) {
          var cssRule = styleSheet.cssRules[j];

          allSelectors.push(cssRule.selectorText);
      }
    } 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 allSelectors;