I believe the bounding box has to fit all the possible ascenders and descenders etc., so the em-height is proportioned within the box to whatever the highest and lowest marks in the typeface are.
But this technique isn’t correcting for some kind of mistake the typographer theoretically made, it’s adjusting for the fact that CSS sizes by the bounding box instead of the em-height. The font itself is unchanged and renders exactly how it’s designed, this just lets us use it in a more intuitive way.
IMO sizing by bounding-box was the wrong move in the original CSS spec, but that’s how digital type renderers have worked going back to the eighties, so the whole thing was probably too entrenched in the first place. And I have no idea if font standards of the day even had em-heights encoded in a way that could be read in font metadata.
Anyway, this whole feature is news to me, can’t wait to try it. Between this and text-box-trim (life changing), I’ve never been happier with web design.
But this technique isn’t correcting for some kind of mistake the typographer theoretically made, it’s adjusting for the fact that CSS sizes by the bounding box instead of the em-height. The font itself is unchanged and renders exactly how it’s designed, this just lets us use it in a more intuitive way.
IMO sizing by bounding-box was the wrong move in the original CSS spec, but that’s how digital type renderers have worked going back to the eighties, so the whole thing was probably too entrenched in the first place. And I have no idea if font standards of the day even had em-heights encoded in a way that could be read in font metadata.
Anyway, this whole feature is news to me, can’t wait to try it. Between this and text-box-trim (life changing), I’ve never been happier with web design.