Deep dive CSS: font metrics, line-height and vertical-align
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Vincent De Oliveira:

Line-height and vertical-align are simple CSS properties. So simple that most of us are convinced to fully understand how they work and how to use them. But it’s not. They really are complex, maybe the hardest ones, as they have a major role in the creation of one of the less-known feature of CSS: inline formatting context.

No kidding, this stuff is complex.