While I'm not a professional typographer, I currently work in user experience, so I might have an answer.
From a cursory glance at the CSS, it's solid black text on a solid white background. This can have slightly too much contrast, which can become tiring to read. Off-black text on an off-white background tends to help I find. Apparently this effect is even worse for dyslexics, and "not-white" is specifically mentioned in: http://www.bdadyslexia.org.uk/about-dyslexia/further-informa...
From a cursory glance at the CSS, it's solid black text on a solid white background. This can have slightly too much contrast, which can become tiring to read. Off-black text on an off-white background tends to help I find. Apparently this effect is even worse for dyslexics, and "not-white" is specifically mentioned in: http://www.bdadyslexia.org.uk/about-dyslexia/further-informa...