Option to keep pages in the ratio of A4 even if the screen has other proportions

I like to use fullscreen view when I read a book in the browser to minimize distractions.

The fullscreen view always fills the entire width of the screen, which even on side-by-side view gets way too wide to read comfortably (see screenshot below) on wide screens.

Can you add an option to let pages always have the standard A4 ratio between width and height even if the screen has other proportions?

I know I can change the margins of the page, but I would like a more permanent solution for different screen sizes. I can’t imagine there are many scenarios where you would want to view a book in this weird, wide format.