FilterStyle: Display/hidden elements by js according to class selectors

FilterStyle.js

function FilterStyle(name, show) {
  "use strict";
  // @see http://stackoverflow.com/a/8630641/599991
  // @see http://www.happycode.info/create-css-classes-with-javascript/

  this.name = name;
  this.show = show;
  if(show !== undefined)
    this.update();
}

FilterStyle.prototype.update = function (show) {
  "use strict";

  if(show !== undefined)
    this.show = show;

  var declaration = 'display: '+((this.show) ? 'initial !important' : 'none' );
  if(this.rule)
    this.rule.style.cssText = declaration;
  else
    this.rule = this.createRule(this.name, declaration);
};

FilterStyle.prototype.findStyleSheet = function () {
  "use strict";
  if (!document.styleSheets || document.getElementsByTagName("head").length === 0)
    return;

  if (document.styleSheets.length > 0)
    for (var i = 0; i < document.styleSheets.length; i++) {
      if (document.styleSheets[i].disabled)
        continue;

      var media = document.styleSheets[i].media;
      var mediaType = typeof media;

      if (mediaType === "string" && (media === "" || (media.indexOf("screen") !== -1)))
        return document.styleSheets[i];
      else if (mediaType === "object" && (media.mediaText === "" || (media.mediaText.indexOf("screen") !== -1)))
        return document.styleSheets[i];
    }
};

FilterStyle.prototype.createStyleSheet = function (){
  "use strict";

  var styleSheetElement = document.createElement("style");
  styleSheetElement.type = "text/css";

  document.getElementsByTagName("head")[0].appendChild(styleSheetElement);
  return this.findStyleSheet();
};

FilterStyle.prototype.createRule = function (selector, style) {
  "use strict";

  var styleSheet = this.findStyleSheet();
  if (styleSheet === undefined)
    styleSheet = this.createStyleSheet();

  var media = styleSheet.media;
  var mediaType = typeof media;

  if (mediaType === "string") {
    for (var i = 0; i < styleSheet.rules.length; i++)
      if (styleSheet.rules[i].selectorText.toLowerCase() === selector.toLowerCase()) {
        styleSheet.rules[i].style.cssText = style;
        return styleSheet.rules[i];
      }

    styleSheet.addRule(selector, style);
    return undefined;
  } else if (mediaType === "object") {
    for (var i = 0; i < styleSheet.cssRules.length; i++) {
      var rule = styleSheet.cssRules[i];
      if (rule.selectorText && rule.selectorText.toLowerCase() === selector.toLowerCase()) {
        rule.style.cssText = style;
        return rule;
      }
    }

    var index = styleSheet.insertRule(selector + " { " + style + " }", styleSheet.cssRules.length);
    return styleSheet.cssRules.item(index);
  }
};