Actually that has changed in modern browsers. That used to be the case, but now both IE and Firefox allow you to increase the size of the whole page (fixed px fonts, images, etc.) Opera already had this option for years, but now you can also do it with IE and Firefox too Browsers are becoming more accessible to the impairedIt does not really matter. With PX you will get the same size on every computer, but the downside is that the user can not increase the font size in there browser. With PT it is the opposite.
Points are for Print
Many Web designers have gotten into the bad habit of using point sizes to define their font sizes on the screen. But points are a print unit of measure, and the Web is usually viewed on the screen. This means that when you specify 14-point type, it might display much larger or smaller than you expect. The most common place this is noticed is between Macintosh and Windows. Macintosh typically displays things almost 25% smaller than the same page on Windows.
Web designers should definitely continue to use points as font-size lengths in their style sheets. But the points should be limited to style sheets for print.
What to Use?
Once you've decided not to use points for your screen style sheets, you need to decide what unit of measure to use. There are two rules of thumb.
Know Your Audience
If you're catering to an older audience, then ems and a larger font size will be more important. If most of your audience browses on a Macintosh and you're designing on Windows, then beware of really small fonts. Decide which is more important, control or accessibility, and use the measure that's right for you.