Many Web developers use points or ems to define font size and serve different sizes to Mac and Windows platforms using JavaScript. See the font sizing article for more details on this topic. Despite the drawbacks, pixels are the best alternative when you need to control font size. But keep in mind that if you use pixels your site may be unreadable by a portion of your audience, and you may be better off designing a site that can handle the larger text a user may prefer. The spacing of a page can also be undermined by form elements.
Forms render at different widths in different browsers.
If you don't account for this variability, you might find that an unexpectedly large text field stretches the top or side navigation in a way that it destroys the cohesive look of a page. As Steve Mulder and Michael Brandt's article discusses, this problem is easily addressed by accounting for form width in your initial design. Most designs will survive when viewed in browsers with smaller form elements. Usually, this will not be too big of a problem on the Macintosh browsers, because forms tend to appear smaller on this platform. When using a text input field, Netscape 4 on the Windows platform displays at the greatest width.