Jakob Nielsen has a good article about web browser usability today. While there is an issue with web designers that favor myopic text sizes, the key issue is with recent versions of IE (post 4.0) that no longer have the text-bigger/text-smaller buttons.
If you ever doubted the utility of usability, check this out:
Compare this awkward, six-step process with the interaction technique required by a design that includes separate buttons for "make text larger" and "make text smaller":1. Click the desired button.
Of course, I am cheating a little: You would still have the initial step of deciding whether you want the text larger or smaller, thus determining which button you'd click. Still, since the entire change-font-size procedure is triggered by your annoyance at trying to read unpleasantly sized text, you already know that you want bigger (or smaller) text by the time you decide to change the size. (The average user doesn't have a mental model of a single "change size" command that is parameterized with the desired direction of change; the user's model includes two actions: "bigger" and "smaller." How the code is implemented is irrelevant for the user illusion which should be designed to match the users' mental model.)
The two-button approach frees users from the cognitive overhead of calculating how big they want the text to be. Just make it bigger. Users don't want to specify exactly how big. They can easily keep clicking the "bigger" button if the initial click doesn't make the text big enough.
Usability is enhanced by single action buttons that move along a uni-dimensional axis in simple steps, as long as each step's result is immediately clear after each click. That's also why the Back button is so precious to users, and why it's used much more frequently than history list navigation.
