There is some useful information here, but I'm a little doubtful about some parts.
Specifying font size in terms of cap-height is ok, and I do agree it's kind of annoying that when using the normal font size metric two different fonts with the same nominal size can actually be visually very different. However I think there are some collections of fonts (collections of font families really) created together where the sizes are supposed to harmonize. For example, Sans / Serif / Mono families of a single collection like the Noto fonts are probably (I say probably because I haven't checked! Someone correct me!) designed together so that, eg, some mono text inline in a sans paragraph will look "right" when the same nominal font size is used for both. If you specify cap-height instead you might subtly break that aspect of the design.
But I agree it generally doesn't work to do that for totally separate families (Noto sans + Courier Prime or whatever), and then cap-height will probably give better consistency and be a better starting point.
> I see no reason to honor the so-called “default line-height”. It’s basically a font designer’s personal preference
Well, the whole design of the font is "a font designer's personal preference". Or to put it the other way around, line height (like letter spacing and other metrics) is part of the design. You might choose to adjust the design by changing these metrics but the values chosen by the font designer are probably a good place to start. It is probably not a good idea (aesthetically) to choose your line height independently of the actual font you're using.
Ultimately, there is a lot of subtlety in font design and typography, and purely geometrical approaches (eg, based on actual computed bounding box) tend to fail. If you want the best sizing and spacing, you probably have to work it out by eye for an individual font in a particular context. Since you have to do that, the fact that sizes are specified using a geometrically arbitrary scaling factor is perhaps not a big problem.
Specifying font size in terms of cap-height is ok, and I do agree it's kind of annoying that when using the normal font size metric two different fonts with the same nominal size can actually be visually very different. However I think there are some collections of fonts (collections of font families really) created together where the sizes are supposed to harmonize. For example, Sans / Serif / Mono families of a single collection like the Noto fonts are probably (I say probably because I haven't checked! Someone correct me!) designed together so that, eg, some mono text inline in a sans paragraph will look "right" when the same nominal font size is used for both. If you specify cap-height instead you might subtly break that aspect of the design.
But I agree it generally doesn't work to do that for totally separate families (Noto sans + Courier Prime or whatever), and then cap-height will probably give better consistency and be a better starting point.
> I see no reason to honor the so-called “default line-height”. It’s basically a font designer’s personal preference
Well, the whole design of the font is "a font designer's personal preference". Or to put it the other way around, line height (like letter spacing and other metrics) is part of the design. You might choose to adjust the design by changing these metrics but the values chosen by the font designer are probably a good place to start. It is probably not a good idea (aesthetically) to choose your line height independently of the actual font you're using.
Ultimately, there is a lot of subtlety in font design and typography, and purely geometrical approaches (eg, based on actual computed bounding box) tend to fail. If you want the best sizing and spacing, you probably have to work it out by eye for an individual font in a particular context. Since you have to do that, the fact that sizes are specified using a geometrically arbitrary scaling factor is perhaps not a big problem.