It would have been great had I known details like this much earlier in my career, but when people talked then about "pixel perfect" designs, I don't think most

It would have been great had I known details like this much earlier in my career, but when people talked then about "pixel perfect" designs, I don't think most people understood then what sacrifices you have to make to do so! Box size calculations should be the same, I think, surely. But text specifically has no such requirements for equal precision. In truth, that type of work would require you to know the output sizes across every supported browser.