The weight works great on a high DPI display, but on older displays, it just doesn't work. Open on a rMBP or iPad, and it's beautiful. I'm guessing the designers are working on a high DPI Mac.

I downloaded all of Google’s font library and installed it locally, means I can read the fonts without having to load them from Google. Still doesn’t help against fonts with 0.5px thin lines.

On the updated version, want to give some actionable feedback:

Not just the weight, the size combined with the weight: 12px in a thin font is too light for a screen, especially done in grey. Going to 16px could really make a difference.

Going with a bigger size gives the scope to use a different, contrasting (perhaps thicker, perhaps thicker and smaller for double-contrast?) font for the headings currently in green.

* Getting older happens to different people at different ages, one of the effects of this means eyes get more temperamental, and this doesn't happen 50+, it happens a lot earlier for a lot of people.

As I said above, the Firefox screenshot looks like a printer that ran out of ink, the Chrome version is slightly better, but look at the letter 'T', that's still wrong, a hairline that falls between pixels. That's only acceptable for text that is not actually intended to be read (thumbnails, mockups, zoomed out stuff, etc).