On 2011/11/09 15:18 (GMT) Barney Carroll composed:
font-size is a little arbitrary: all measurable aspects of the glyphs 'M' or 'm' (or indeed any other glyph) set in Arial are smaller than 12px; with Calibri they are smaller still.
Fonts sized in px are _completely_ arbitrary once the design escapes the designer's environment. Fonts so sized disregard the optimal size as established by the default size set in the viewer's UA.
Anything as small as 12px on any of my systems is illegible, but you are right that Vista-generation M$ fonts typically render smaller than traditional/common web fonts.
Practically speaking:
I don't see a convincing argument for having layout elements infer their size based on font-size. I tried this once, and ended up with 30em as a total width that worked well on desktop with my base font-size of 12-pixels — but for those people who need higher resolution base heights, this is a disaster: the layout requires massive panning left and right to be able to make sense of it (especially on the phone, which has pretty limited screen size). Better keep the font-size based on the em, but keep the layout measurements fixed or fluid based on fractions of the viewport (%).
If you're not interested in any of these aspects, use pixels: they're precise, unambiguous, and suffer none of the headaches of the cascade.
On the contrary, px are completely ambiguous as soon as the viewer is not the designer looking at his own work on his own system.
To start with, there are two kinds of px, device px, and CSS px, the relationship between which is unpredictable. Device px vary wildly, from under 40 ppi to more than 300.
The relationship between px and legibility/usability is also unpredictable, since even when assuming CSS px the relationship between legibility/usability still unconditionally disregards whatever size the visitor's browser defaults to, the presumptively optimal size for the majority of any page's text in that viewing environment.
When you size with em, or better yet rem, you're able to maintain the design relationships among elements while embracing physical sizes based on what's optimal in the viewing environment, respecting user settings instead of entirely disregarding them.
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