How can I modify web content on a mobile browser?

I want to build a small extension for major browsers that modifies the HTML and CSS of certain pages. On desktop, this can be accomplished easily. On mobile, most browsers don’t allow content-modifying extensions. Other than creating my own browser or a specialized mobile app, how might I achieve this goal with low friction for the user?

(This may be better suited for a different stack exchange forum. I’m not sure)