Opera/Chrome, how to fully remap fonts?

Question

css

My eyesight is +1 (plus one) and I hate serifs because they are unreadable. I assigned the font family on the browser properties page to all 4 of them (std, serif, sans, monospace), which I always set to Arial/Arial/Arial/Courier or Segoe/Segoe/Segoe/Courier. This setting is available in all modern browsers, including non-chrome-like ones.

Why on some sites , including some articles on Habré, and especiallyon news sites (they have an official order there, to piss off the visitor?) does this setting not work? Many sites are protected from copying text from them, and from injecting styles / code. How to read it? Why does some special designer decide for me which font is better for me to read, without asking? I'm my own boy, my own, and I'm not interested in someone else's ideas about the perfect type. I told the system: I don't want to see serifs on the font. Generally. Everything.

AddCSS and similar extensions don't help. Honestly, the first emotion from such a site is to press Ctrl + W and not remember. Web designer, don't eat, think.