Issue 4676 - Replace non-standard CSSStyleSheet.addRule() call by CSSStyleSheet.insertRule()

include.preload.js

Index: include.preload.js
===================================================================
--- a/include.preload.js
+++ b/include.preload.js
@@ -575,17 +575,18 @@ ElemHide.prototype = {
     // Safari only allows 8192 primitive selectors to be injected at once[1], we
     // therefore chunk the inserted selectors into groups of 200 to be safe.
     // (Chrome also has a limit, larger... but we're not certain exactly what it
     //  is! Edge apparently has no such limit.)
     // [1] - https://github.com/WebKit/webkit/blob/1cb2227f6b2a1035f7bdc46e5ab69debb75fc1de/Source/WebCore/css/RuleSet.h#L68
     for (var i = 0; i < selectors.length; i += this.selectorGroupSize)
     {
       var selector = selectors.slice(i, i + this.selectorGroupSize).join(", ");
-      this.style.sheet.addRule(selector, "display: none !important;");
+      this.style.sheet.insertRule(selector + "{display: none !important;}",
+                                  this.style.sheet.cssRules.length);
     }
   },
 
   apply: function()
   {
     var selectors = null;
     var propertyFiltersLoaded = false;