Web Design in a Nutshell, 3rd Edition

The problem with pixels

You may be thinking that because elements on web pages are measured in numbers of pixels, and because pixels are considered a relative measurement, that they are the answer to all font size problems. It would be nice if they were. For some designers, control over text at the pixel level is intoxicating.

Unfortunately, there are a few reasons why pixels have come to be shunned for text size. We know that all pixels are not created equal, so that means that what is tidy yet readable on your monitor may require a magnifying glass on someone else’s screen.

On most current browsers, starting with Internet Explorer 5 for Macintosh in March 2000, that is not a problem, because users have a “text zoom” function that allows them to increase the size of text regardless of the style sheet settings. Ironically, Internet Explorer for Windows (Version 6 and earlier) does not allow text zoom on text specified in pixels (it will resize text set in ems and percentages). IE 7 (in beta as of this writing) promises a zoom function on pixels, but for the time being, there is a significant percentage of users who cannot override pixel size settings. This is a big no-no in terms of accessibility.

Recommendation: If accessibility is important to you (and it should be), avoid using px measurements for text until IE 5 for Windows and IE 6 for Windows are just a memory.