I want to see the text either the way you envisioned it, or at my default size. If you use pixels, great! I get to see your masterpiece. If you use ems, you’re tapping into my preference for unstyled sites and getting it wrong.
I see what you’re getting at: Pixel sizes are optimal for the designer, and for users who wish to see exactly what the designer intended. Default sizes are optimal for the user*. Em/percent sizes other than 1em/100% are optimal for neither.
However, I don’t see this as a big problem; if the site is to be able to look good and be fully usable with the users default font size, it has to accommodate for a wide range of sizes, since not all users have the same default. This means that it should also be able to look good and be usable with a font size that is proportionally bigger/smaller than your default (80% of your default is 100% of someone else’s default).
And if a site is NOT able to look good and be fully usable with any other font sizes than the designer’s, then it’s only optimal to those who are able to comfortably read in that size, right?
Here comes the asterisk: *Default sizes are only optimal for those users who have set their default size, and it appears to me that few users have, or are even aware that they can. I’d be happy to be proven wrong, though.
To summarize the pros and cons of different font sizing methods:
+ Allows the designer to create a subjectively perfect design that users are able to see as intended, provided they don’t override the font-size with e.g. !important directives.
- Suboptimal font sizes for those who don’t have the same taste and eyesight as the designer.
- Does not allow users of IE to change the font size from site to site. (You might say that they could just use e.g. !important, but even assuming users know how to do that, they might only have trouble reading a FEW sites, and might have no desire to override ALL sites’ settings because of this.)
Default size pros:
+ Always optimal font size for those users who’ve set a default size.
Default size cons:
- Suboptimal for those who haven’t, and they are probably many.
+ Provided it’s sensible (like, never smaller than 0.8em/80%), and provided that users who set default sizes realize that the default size should be a BIT bigger than “just about readable without squinting”, it should be CLOSE to optimal (VERY close or even spot on for a lot), no matter if the users prefer 10px or 100px text, no matter if they’ve set a default size or not, without any modifications from site to site.
- Well, it’s still suboptimal.
Feel free to add to the list, and remember (all of you) that we’re only talking about base font sizes; it’s never a bad thing to use relative font sizes e.g. to make headings (sensibly) bigger than body text. And ems vs pixels vs percents for sizing other things than fonts is an entirely different subject.
Now, I really do think that the best solution would be if everyone just set their default sizes so that they could comfortably read unstyled text (and leave some room for smaller text like copyright notices and such), and that designers just left the base font size at 100%. But unless both these things happen at once, there’s going to be suboptimal font sizes for a lot of users at a lot of sites. And while it might be fair to ask web designers to do their part, I don’t think it’s fair to ask users to “please set your font sizes to show web designers we mean business, even if it means that text becomes too small on sites that don’t respect your settings”.
There is the argument that users who go to the trouble of setting a default font size should be rewarded, and that those who don’t take advantage of their browsers configurability have themselves to blame. I think this argument is mostly valid, but I also think there’s validity to the counter-argument that modern browsers often provide quick ways to specify both a fixed font size (a la !important) and a minimum font size, making sure that text is never too small, and that hardcore power users can create their own stylesheets.
So, my opinion is still that no method of font sizing is perfect, but today, ems/percents (or keywords, for that matter) come the closest, with default sizes a close second. Pixel sizing comes a long way down, because its benefit is only a benefit to the designer, to those users who share the designers taste and eyesight, and to those users who are more interested in looking at a beautifully designed site than actually reading or using it.