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.

Thanks - I'm really grateful for the feedback - and would like it if you have any more.

I'll be changing the base font to 14px and darkening the grey to #222 quite soon.

I'll also look at 16px but this needs some additional design work.

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).