event_rules

chrome.declarativeContent.onPageChanged.addRules([{
  actions: [
    new chrome.declarativeContent.ShowPageAction()
  ],
  conditions: [
    new chrome.declarativeContent.PageStateMatcher(
        {css: ["video"]}
    )
  ]
}]);

{
  "name": "Sample extension",
  "event_rules": [{
    "event": "declarativeContent.onPageChanged",
    "actions": [{
      "type": "declarativeContent.ShowPageAction"
    }],
    "conditions": [{
      "type": "declarativeContent.PageStateMatcher",
      "css": ["video"]
    }]
  }],
  ...
}