Philippe Wittenbergh wrote:

> A modern OS / browser will do the job for you.

But most users, or (to be cautious) at least a non-negligible share like 40% 
of users, seem to be using an OS / browser that in non-modern by your 
implicit definition.

> p { font-family: font-a, font-b, font-c, serif;}
>
> Gecko, WebKit, Opera, and IE 8+ will look for the glyphs in font-a,
> if that doesn't have the coverage [*], the browser looks at font-b,

I'm not sure whether that happens even on those browsers in all situations. 
Even if it does, the situation is far from perfect.

> The good news for you: your first choice is installed by default on
> OS X and Windows Vista +. For Linux, throw in DejaVu Sans

Pardon? Arial Unicode MS is normally distributed along with Microsoft 
Office. You might get the impression of distribution with Vista because 
quote often a new PC with Vista comes with a pre-installed trial version of 
Microsoft Office.

Anyway, Arial Unicode MS is far from universal. Moreover, it is a single 
typeface - no italics, no bolding (though browsers may apply algorithmic 
italics or bolding to it, with typographically poor results inevitably). But 
the problems don't end here.

> I specify 'Helvetica
> Neue' as the font of choice on OS X; but that font doesn't have
> coverage for some romanized characters (e.g ō), I thus specify a
> fallback: 'helvetica', that has close-to-the-same metrics & look.

That might be a good example in some sense, but it is a fairly limited case. 
It may help on OS X platforms, but what would happen in a more typical 
situation? The fallback of 'helvetica' is simply ignored. You could use e.g.
font-family: 'Arial Unicode MS', Arial, sans-serif
but what would happen when Arial does not contain glyphs for the characters 
needed? Moreover, even Arial Unicode MS might lack them, since different 
versions of that font (typeface) come with different character coverage.

Even if you carefully select a fallback font that is compatible with the 
primary font (and usually you can't do that very carefully, as the options 
are so limited in practice), a mix of fonts tends to produce bad results at 
least when a word contains letters from different fonts. For separate 
symbols, a mix is not that bad, if the fonts are roughly similar.

The morale is that fallback fonts are nothing you could count on. They may 
help at times, but basically you should select the primary font so that it 
is suitable for your needs and widely enough available. Depending on the 
content and purpose, different compromises need to be made. For example, if 
your material contains a large repertoire of special characters, you 
probably need to accept the consequence that many users won't see the page 
properly (though most can), due to use of a font like Arial Unicode MS.

Jukka 

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