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I've been trying to get to the bottom of how to best set vertical metrics in web fonts. These issues go way back, all the way to the creation of the TrueType format, which had separate Windows and Mac fields. Setting them wrong will lead to clipping, strange line spacing, and inconsistencies between browsers - sadly very familiar outcomes for people using web fonts. I'd like to make all that go away, and, just as important, make life easier for font designers, so they can just set the metrics and be confident they're right.
To _really_ understand vertical metrics on the web, I created some test cases and tested on a lot of browsers. I also came up with some draft recommendations, generally similar to the ones Typekit has been promoting.
My recommendations, along with the raw source and screenshots from the test cases, as well as references to source materials (many typophile threads were quite enlightening) are here: http://code.google.com/p/googlefontdirectory/wiki/VerticalMetricsRecomme...
That's a community wiki page, and I welcome contributions, as I'm sure people will have more insight. If you've got something to add (and are not a spammer :), let me know and I'll add you. And, of course, I'm hoping for some spirited discussion here.
One of the other things that emerged from my testing is that people should really be specifying CSS line-height explicitly, rather than defaulting to line-height: normal. The actual spacing behavior of the latter is extremely complex and inconsistent from browser to browser. When you specify line-height explicitly, things get a lot better, but baseline positioning can still be wobbly.
And I'm very happy if people have questions, or feel there's something unclear in that document. I've been spending quite a lot of time digging into this and feel it's a good time to spread what I've found more widely.