The implications of this are significant. Firstly, it guarantees that it is essentially impossible to have text look identical on Macintoshes and Windows based systems. But if you embrace the adaptability philosophy it doesn't matter . If you are concerned about exactly how a web page appears this is a sign that you are still aren't thinking about adaptive pages. One of the most significant accessibility issues is font size.
Small fonts are more difficult to read.
For those of us with good eyesight, it can come as a shock that a significant percentage of the population has trouble reading anything below 14 point times on paper . Screens are less readable than paper, because of their lower resolution. That doesn't help those whose sight is even less strong. Don't use points. This allows readers to choose the font size which suits them.