Add position for the inert attribute. (#371)

Mark it as worth-prototyping as per this comment[1] and following.

Closes #4288, let's continue the more technical discussion somewhere
else, like in whatwg/html#5650.

[1]: #174 (comment)

activities.json

@@ -1178,6 +1178,18 @@
     "title": "enterkeyhint attribute",
     "url": "https://html.spec.whatwg.org/multipage/interaction.html#attr-enterkeyhint"
   },
+  {
+    "ciuName": "mdn-api_htmlelement_inert",
+    "description": "Allow arbitrary DOM subtrees to become inert",
+    "id": "inert-attr",
+    "mozBugUrl": "https://bugzilla.mozilla.org/show_bug.cgi?id=921504",
+    "mozPosition": "worth prototyping",
+    "mozPositionDetail": "A high-level tool for authors to achieve some of these use cases while getting a11y right seems useful",
+    "mozPositionIssue": 174,
+    "org": "WHATWG",
+    "title": "inert attribute",
+    "url": "https://github.com/whatwg/html/pull/4288"
+  },
   {
     "ciuName": null,
     "description": "Proposal to add various output attributes on HTML's <input> elements that would allow developers to declare some conversions the browser could do to, for example, images and videos.",