It has everything to do with the chosen font and how font-weight: 300 is displayed. The font-family is set to sans-serif and on my Linux system, it selects DejaVu Sans ExtraLight, which looks like that.
This is actually one reason to use webfonts, since you can be reasonably(ish) sure it looks more or less the same for everyone.
> This is actually one reason to use webfonts, since you can be reasonably(ish) sure it looks more or less the same for everyone.
This seems to be a general conflict between "it should look like the person who made it intended it" and "it should look like I want it to". For instance prescribing fonts and colors gives you a nice uniform look, but causes problems with dark themes or wanting other fonts, not to mention hijacking scrolling for fancy effects.
Similarly, clientside decorations make apps like chrome look similar on all OSs, but also out of place on all of them.
If you're writing a blog post, IMO you shouldn't care about how the site looks, other than it being easy to read. The best way to make it easy to read is black text on a white background and no clutter (the latter, to be fair, is something this site does well).
This is actually one reason to use webfonts, since you can be reasonably(ish) sure it looks more or less the same for everyone.