I really do not think it should be up to web page designers to dictate what font you view a web page with. There are websites you visit and you can hardly read content because someone specified some insane font style and size for the page, which works well if the viewer's web browser is left in defaults. If you in fact have the fonts and can do the sizes specified, trouble happens. I wish I could just avail a few fonts to a browser and force every website I visit to be displayed in just those few fonts as desired.
Also, don't developers ever test their sites across browsers any more? I've come across a few websites that work well only in IE or Firefox and suck in other browsers. Even if you develop for IIS only, it's imperative you test on other browsers ... unless you don't care about visitors that exercise freedom of browsers. At least have a disclaimer if you have to go that way.
For the rest of us that customize our browsers to the tee, including specifying a font and style, it becomes an annoyance. Preferences change often, but I like "Californian" and "Courier New" at 10pt for my pages - small font. If I were to visit a site that attempts to reduce the font size by 3 points (relative), for example, the text would be unreadable. This is where Firefox excels at giving control to the user - you can specify a minimum font and the browser ensures that all font sizes are always above that size. I don't know anywhere in IE where this can be done. In Firefox, you also have to unselect the option that allows pages to choose their own fonts.
I always recommend that when you design web pages, the bulk of your content shouldn't have restrictions on what font styles and sizes users should use. Most users customize these two things, and you'd be abusing your authority as an author to restrict that freedom. It's understandable for fonts that must fit on images or certain fixed spaces such as menus and ads. Developers that use templates that calculate font sizes based on available space, etc, are prone to this problem. I discourage using relative font sizing (e.g. <font size="-3">) for text that can be manipulated by browsers. User preferences with fonts and styles can also mess up your menus and such that depend on fixed spaces. For such, images are the way to go - to promote consistency.