Use Chrome Extension to Modify Google Search Result Page

Did you add jQuery to your content_scripts in manifest?

If you use jQuery, you must write manifest.json like this:

 "content_scripts":[
        {
            "matches":["http://www.google.com/*"],
            "js":["jquery-1.9.1.min.js", "contentscripts.js"]
        }
]

OK, after reading the page source of Google search result page I think I know what the problem is:

Google loads search result with AJAX, so, when you change query words and search again, the page DOES　NOT refresh, that's why you can not get any DOM elements in the search results.

That means you have to add an event listener for DOMNodeInserted.

Code is like this:

function findH3(){
  $('h3.r').append('<b>a</b>')

}

searchResultArea.addEventListener('DOMNodeInserted', findH3);