There are only two ways to avoid these problems. Most designers hate this idea because we're used to the absolute control we've enjoyed in print. But it's the most compatible and foolproof approach. Sometimes you absolutely must control font sizes. In these cases, you need to specify pixels in your Style Sheet.
Pixels are pixels regardless of screen resolution, platform, browser choice, or browser version.
If the Site Fits . . . You finish your site's master template, sigh with satisfaction, and inform your Windows- using client that the layout is ready for final approval. The Windows task bar appears by default at the bottom of the screen, while Mac OS reserves the right side of the screen for icons representing your hard drive, saved files, and aliases. Your client sized his browser window so he can see crucial areas of his desktop, which means his browser window wasn't as deep as yours. Similar glitches crop up when you design only for your monitor size.