It might not be impossible or even difficult to read for you or I, but remember that every OS displays fonts a little differently, indeed some browsers may display differently, and different monitors display a little differently. People could be having a vastly different experience from what you or I have.

There's a reason sans-serif black-on-white is the default, and why people ask for it. It's easy to read no matter what. That's why Readability exists. And for what it's worth, at first glance I find this page difficult to read as well on a 1080p 15" monitor. The font makes me have to focus on one line and one word at a time, where in stock sans serif I can skim. That's what happens when you try to translate text that works fine in a novel and put it on a computer screen. You would never see a newspaper or a textbook printed in this fashion. Because it's harder to read.