declarativeContent

It was used for extensions that were only relevant on specific types of pages.

Examples:

• A "video downloader" extension that would only enable its icon on pages containing a <video> tag.
• An RSS feed reader extension whose icon would only appear on pages that had a <link rel="alternate" type="application/rss+xml"> tag.

{
  "name": "My Declarative Content Extension",
  "version": "1.0",
  "manifest_version": 2,
  "permissions": [
    "declarativeContent"
  ],
  "background": {
    "scripts": ["background.js"],
    "persistent": false
  },
  "page_action": {
    "default_title": "I'm active on this page!"
  }
}

Note: In Manifest V3, this API is not available. The recommended alternative is to use scripting with activeTab or specific host_permissions to check page content when the user clicks the action icon, and then use chrome.action.setIcon() or chrome.action.enable()/disable().

Why is it not risky?

This was a privacy-focused permission available in older extensions. It allowed an extension to show its icon only on certain pages (e.g., a video downloader that only appears on YouTube) without actually reading the page's content.

Because the browser itself checked the page content instead of the extension, the extension couldn't access any of your data. This made it very safe to use. Modern extensions achieve similar results using different methods.

// background.js (Manifest V2)

chrome.runtime.onInstalled.addListener(function() {
  chrome.declarativeContent.onPageChanged.removeRules(undefined, function() {
    chrome.declarativeContent.onPageChanged.addRules([{
      conditions: [
        new chrome.declarativeContent.PageStateMatcher({
          // Show the page action if the page has a <video> element
          css: ["video"]
        })
      ],
      actions: [ new chrome.declarativeContent.ShowPageAction() ]
    }]);
  });
});