Member Rara Avis
As Chris suggested, relative fonts sizes are indeed designed to improve accessibility for the visually impaired. And that's a good thing, both for you and for them. I've heard estimates that go as high as eight percent for the number of surfers who suffer from visual impairment. That's a lot of people to arbitrarily ignore.
However, to really answer the question, we need to be more specific. Could we have a little history meandering music, please?
In the beginning there was HTML. A fixed font size ranged from 1 (teeny tiny) to 7 (pretty massive), with 3 being the default if the designer failed to specify a size. We also had relative font sizes, based on the BASEFONT tag. For example, at the top of the page I might say <BASEFONT COLOR="#000080" SIZE=3> and thus set my basefont to the same default size of 3. If I later said <font size=+1> I would essentially be setting it to 4, just as <font size=-1> would set it to 2. The idea behind the basefont was a good one. I could design my page and then quickly change the size of ALL the text by changing just the definition of the basefont. It was a good idea, but no one ever used it.
This page (as with all the pages in the forums) uses HTML's fixed font size. Not relative. Yet, if you go to View/Text in your browser, you should find that you can STILL change your view of the font size. If you're using the font size built into HTML specifications, it really doesn't matter whether your use fixed or relative. The browser can and will adjust either for the visually impaired. This is a good thing!
In the beginning there was HTML and it was good. But, uh, it wasn't good enough. So, some really smart people came up with Cascading Style Sheets (CSS) to give designers greater control. And it TOO has both fixed and relative font size specifications.
Fixed size is what everyone seems to be using. If I set my font size to 11px (pixels), that is exactly what the browser will display. You can change your View/Text settings all day long and you are still going to see eleven pixels. That is NOT a good thing, at least not for the visually impaired. For the anally retentive, I guess it's cool, though.
While the relative font size specifications in CSS are very different from the old 1 to 7 specs in HTML, they are also quite similar in that we have seven different sizes.
Most browsers (and certainly IE) will display xx-small text as equivalent to 10px and xx-large text as equivalent to 48px. Unlike CSS's fixed font size specifications, however, these CAN be adjusted by the individual user. It gives BACK to the visually impaired the ability to alter the way your web page looks to them. A very good thing.
Obviously, CSS absolute font sizes gives the designer more control. Absolute control, in fact. None of the seven relative font sizes, for example, work out to eleven pixels and if you feel you need eleven pixel text, the only way to get it is with 11px. In my opinion, however, you should have a VERY compelling reason to ever use absolute font sizes. It's not much different than taking out all the handicapped parking spots at your business or removing the ramp at your school.
There are actually a lot of other things to consider when building a web site with accessibility in mind. For those interested, there's a program called Bobby at http://bobby.watchfire.com/bobby/html/en/index.jsp that can be a big help. Enter the URL of your home page (or any page on your web site), and Bobby will read through your HTML and report back what it sees as potential accessibility problems. Unfortunately, in my opinion, Bobby is a little TOO picky and that can make the report hard to read. To really understand what Bobby tells you will require a bit of study, but the report can nonetheless lead you in the right direction. Give it a shot!
Uh, you can turn off that history meandering music now, please Ö