There is a plague of tiny
sans-serif fonts being used for body text on the web. I find sans-serif to be difficult enough to read without it being tiny as well, and am rather certain the prevalence of this type of text is one of the primary reasons people find it uncomfortable to read book-length material on computer screens; that and cheap low-ppi (
pixels per inch) monitors. Even I find sans-serif fonts more readable on low-ppi monitors, as such monitors reproduce serifs poorly, but I can't read for a long period of time on those either.
There are
hacks available to change sans-serif fonts on-screen to larger or (as I use in cases where I will be reading a lot) serif fonts using CSS. For most sites which I only browse occasionally I find
NoSquint more practical. This handy Firefox plugin allows you to conditionally resize text on pages in an easy and site-specific way, and it remembers your size settings on the next site visit.