Add position for the inert attribute.

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 the specification issue.

[1]: mozilla#174 (comment)

activities.json

@@ -1213,5 +1213,17 @@
     "org": "WHATWG",
     "title": "timezonechange event",
     "url": "https://github.com/whatwg/html/pull/3047"
+  },
+  {
+    "ciuName": null,
+    "description": "Allow arbitrary DOM subtrees to become inert",
+    "id": "inert-attr",
+    "mozBugUrl": null,
+    "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"
   }
 ]