Hi again, Michelle.
Thanks for the thorough reply. However, I’m not sure that I’ve conveyed the problem accurately enough, and I think there’s been some confusion.
I’m aware that the single-quoting of fonts is supported in the real-world clients we’ve both listed.
But this single-quoting doesn’t appear to be supported in the EoA simulations of these clients.
And by that I don’t mean that it’s not displaying the fonts I specify in single quotes. I mean that the single quotes themselves are causing non-realistic results in some simulations.
To make sure, I compiled a simple test e-mail and ran it through your Acid Tests.
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First up; how it should look, as illustrated by your AOL Web simulator [among others]:
http://www.plasticle.co.uk/test/eoa/Acid-Test_AOLWeb-CORRECT.gif
Secondly; how it rendered in your Live Mail simulator [among others]:
http://www.plasticle.co.uk/test/eoa/Acid-Test_LiveMail.gif
Not even the Thunderbird 3 simulation comes out looking too clever, either:
http://www.plasticle.co.uk/test/eoa/Acid-Test_Thunderbird3.gif
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So as you can see, some client simulations do NOT handle single quotes in font stacks in the way that they should.
Furthermore, they don’t even fall back to the generic-family types that I’ve declared at the end of the stack. Some render a sans-serif like Arial, while some others render in good old TNR. Neither of which is specified anywhere in the stack, directly or generically.
And this, sadly, is the potential game-breaker. If the simulations don’t replicate the real-world clients *exactly*, then what’s the point in using them?
I do hope I’ve conveyed the issue a bit better this time around. If I can’t do it with screenshots, then gawd help me :D
And to end on a more positive note, your font-stack toggling would be very neat indeed. How you’d achieve it I don’t know, but it’s a great idea 
Thanks again,
- Rich at Plasticle.