![](https://i0.wp.com/www.usablepatterns.net/wp-content/uploads/2019/06/art-big-data-bright-158826-e1559425683345-1024x422.jpg?resize=1024%2C422&ssl=1)
Bookmarklets are a great way to add functionality without writing a full-blown browser extension. They should work in all the major browsers (even Opera which I love).
I have created simple bookmarklets to adjust screensize. It works when only one tab is open, but I can check the responsiveness of my design quickly.