The font stack is just "Helvetica, sans-serif" but the weight is set to 100 which is extremely light. So if you have a very light variant of Helvetica installed it might be unreadable.
Edit: or if your default sans-serif font has a super light variant installed, it might look very different from what was intended.
100 is not a font weight that makes sense for a large body of text no matter what the font. We call 300/400 "normal" for a reason.
100 is a "sometimes food" for that giant header or over-sized pull quote.
Designers that insist that entire documents/web-pages must be "light and airy and always 100 font-weight" have A) never struggled with contrast or accessibility issues, B) maybe don't understand typography. "light and airy" also means "easily blows off the page and is unreadable".
Edit: or if your default sans-serif font has a super light variant installed, it might look very different from what was intended.