Unsharp text on mac os 10 / x

hi every one,

i’m developping a website, i’m doing this on a mac with os 9. no problem with that, it all works fine, also when i test on my pc

however, my client works also on a mac with mac, but one with mac os 10.

en in the texts on the site some parts are sharp and others not.
terrible and he ain’t happy with that. he doesn’t wants the site like this online.

but how can i solve this, on mac os 9 and pc’s it all looks fine.

if you want to check the site yourself: