I need to do a web-extension (add-on) that can manipulate DOM elements, existing events and execute scripts in the context of a page in a website.
But do I need this to happen in background without the page being explicitly in the user's browser, ie do I want to manipulate a page without opening it in a flap or window?
One way to do this is to use iframe , below an example of how to do it (examples using jquery).
Create the iframe and put the appropriate style to make it invisible to the user:
let iframe = "<iframe id='iframe' src='https://pt.stackoverflow.com' style='display:none;visibility:hidden' />";
$("body").append(iframe);
To manipulate elements within the iframe, use .contents :
let page = $("#iframe").contents();
Now just look for the elements:
page.find("input");