use WeakSet, add back safari 9

blocking-elements.js

@@ -60,10 +60,10 @@
 
       /**
        * Elements that are already inert before the first blocking element is pushed.
-       * @type {Set<HTMLElement>}
+       * @type {WeakSet<HTMLElement>}
        * @private
        */
-      this[_alreadyInertElements] = new Set();
+      this[_alreadyInertElements] = new WeakSet();
     }
 
     /**
@@ -241,15 +241,15 @@
      * doesn't specify if it should be reflected.
      * https://html.spec.whatwg.org/multipage/interaction.html#inert
      * @param {!Array<HTMLElement>} elements
-     * @param {Set<HTMLElement>} toSkip
-     * @param {Set<HTMLElement>} toKeepInert
+     * @param {WeakSet<HTMLElement>} toSkip
+     * @param {WeakSet<HTMLElement>} toKeepInert
      * @private
      */
     [_inertSiblings](elements, toSkip, toKeepInert) {
       for (let i = 0, l = elements.length; i < l; i++) {
         const element = elements[i];
         const children = element.parentNode.children;
-        const inertedSiblings = new Set();
+        const inertedSiblings = new WeakSet();
         for (let j = 0; j < children.length; j++) {
           const sibling = children[j];
           // Skip the input element, if not inertable or to be skipped.
@@ -326,15 +326,15 @@
      * Returns the distributed children of the element's shadow root.
      * Returns null if the element doesn't have a shadow root.
      * @param {!element} element
-     * @return {Set<HTMLElement>|null}
+     * @return {WeakSet<HTMLElement>|null}
      * @private
      */
     [_getDistributedChildren](element) {
       const shadowRoot = element.shadowRoot;
       if (!shadowRoot) {
         return null;
       }
-      const result = new Set();
+      const result = new WeakSet();
       let i;
       let j;
       let nodes;

dist/blocking-elements.js

@@ -93,10 +93,10 @@ var createClass = function () {
 
       /**
        * Elements that are already inert before the first blocking element is pushed.
-       * @type {Set<HTMLElement>}
+       * @type {WeakSet<HTMLElement>}
        * @private
        */
-      this[_alreadyInertElements] = new Set();
+      this[_alreadyInertElements] = new WeakSet();
     }
 
     /**
@@ -318,8 +318,8 @@ var createClass = function () {
        * doesn't specify if it should be reflected.
        * https://html.spec.whatwg.org/multipage/interaction.html#inert
        * @param {!Array<HTMLElement>} elements
-       * @param {Set<HTMLElement>} toSkip
-       * @param {Set<HTMLElement>} toKeepInert
+       * @param {WeakSet<HTMLElement>} toSkip
+       * @param {WeakSet<HTMLElement>} toKeepInert
        * @private
        */
 
@@ -329,7 +329,7 @@ var createClass = function () {
         for (var i = 0, l = elements.length; i < l; i++) {
           var element = elements[i];
           var children = element.parentNode.children;
-          var inertedSiblings = new Set();
+          var inertedSiblings = new WeakSet();
           for (var j = 0; j < children.length; j++) {
             var sibling = children[j];
             // Skip the input element, if not inertable or to be skipped.
@@ -410,7 +410,7 @@ var createClass = function () {
        * Returns the distributed children of the element's shadow root.
        * Returns null if the element doesn't have a shadow root.
        * @param {!element} element
-       * @return {Set<HTMLElement>|null}
+       * @return {WeakSet<HTMLElement>|null}
        * @private
        */
 
@@ -421,7 +421,7 @@ var createClass = function () {
         if (!shadowRoot) {
           return null;
         }
-        var result = new Set();
+        var result = new WeakSet();
         var i = void 0;
         var j = void 0;
         var nodes = void 0;

package.json

@@ -62,6 +62,11 @@
         "microsoftedge",
         "14"
       ],
+      [
+        "OS X 10.11",
+        "safari",
+        "9"
+      ],
       [
         "OS X 10.12",
         "safari",