If JS is turned off even in vanilla Firefox, and I apply modifications to the DOM (like CSS mods, zooming in etc.) after the website has finished loading, I can’t see how a website or ISP could detect what the user is doing in the DOM.
I know if you, for example, hide an element in CSS before it is fully loaded (such as the sidebar), the browser may skip downloading resources (such as icons) associated with the element. This would distinguish your web traffic patterns from other users. That’s why I wait till the page is fully loaded. I’m also careful to not trigger CSS media queries which can be set up to connect to a remote URL if triggered (or remove them first if they will be triggered).
I think the above should be enough to avoid distinguishing myself by my web traffic. Do you see any way I could be distinguished with only CSS & HTML?