Create a mechanism define declarative rules via the extension manifest.

chrome/common/extensions/docs/templates/articles/manifest/event_rules.html

+<h1>event_rules</h1>
+
+<p>
+The <code>event_rules</code> manifest property provides a mechanism to add rules that intercept, block, or modify web requests in-flight using <a href="../declarativeWebRequest">declarativeWebRequest</a> or take actions depending on the content of a page, without requiring permission to read the page's content using <a href="../declarativeContent">declarativeContent</a>.
+</p>
+
+<h2 id="translation">Translating rules from javascript to manifest</h2>
+
+<p>
+The following defines a rule to display a page action if the current page has a video css tag in javascript:
+</p>
+
+<pre data-filename="example.js">
+chrome.<b>declarativeContent.onPageChanged</b>.addRules([{
+  actions: [
+    new chrome.<b>declarativeContent.ShowPageAction</b>()
+  ],
+  conditions: [
+    new chrome.<b>declarativeContent.PageStateMatcher</b>(
+        <b>{css: ["video"]}</b>
+    )
+  ]
+}]);
+</pre>
+
+<p>
+This is the same definition in the manifest:
+</p>
+
+<pre data-filename="manifest.json">
+{
+  "name": "Sample {{platform}}",
+  "event_rules": {[
+    "event": <b>"declarativeContent.onPageChanged"</b>,
+    "actions": [{
+      "type": <b>"declarativeContent.ShowPageAction"</b>
+    }],
+    "conditions": [{
+      "type": <b>"declarativeContent.PageStateMatcher"</b>,
+      <b>"css": ["video"]</b>
+    }]
+  }],
+  ...
+}
+</pre>