This is a feature that I have implemented on my site, and I think it'd be a good addition.
Let the user configure his/her font size. I have good vision, but some people are nearly blind and would probably like to see message text at a larger base size. You could have three settings, equal to the standard HTML 'FONT' tag sizes of 2, 3, and 4. These could be mapped to "Small", "Medium/Default", and "Large". If you were feeling particularly nice, you could even go smaller or larger.
I say this for a couple of reasons - first, it should be pretty simple to implement. Second, it would help those of us with 'alternative' platforms to compensate for the tendency of certain browsers/operating systems to render fonts at different sizes. Third, as I mentioned previously, those with bad eyesight will thank you.
If you have extra time, you could even add the capability to select from a sysop-definable list of colors to use for the text. Although I doubt letting the users choose the entire color scheme of the forum would be suitable for those of us who design the UI, allowing them to change the color of the text would be nice (esp. if you could define a seperate color for quoted text, if you were to add that feature).
Anyhow, just a few thoughts... Have fun. :>
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