Increase font size on any website

I’m trying to create a browser extension that allows users to change the font size on any given page. I’ve already researched increasing font-sizes using JavaScript, and it always says to add stuff like ‘font-size: 2em;’ to the body style. This works, however it doesn’t work for every website.

I’m hoping I can do something similar to the ‘Bigger Text’ button like shown here.

(Click on the icon in the top right corner)

Any pointers or suggestions would be greatly appreciated.

Thank you.