Python Flask Modifying Page after loaded
Short answer: you can't.
Longer answer: once you have "sent the page" (that is, you have completed a HTTP response) there is no way for you to change what was sent. You can, however, use JavaScript to make additional HTTP requests to the server, and use the HTTP responses to modify the DOM which will change the page that the person is looking at. There are many ways to make a live chat feed, all of which are too complicated to put in a single Stack Overflow answer, but you can be sure that they all use JavaScript.
I have a question about using Flask with Python.
Lets say I want to make a website for some mod I'm making for a game, and I want to put in a live chat feed, how would I go around modifying the contents of the page after the page has been sent to the person?