> Libraries don't always provide font metrics, and when they do, these metrics are not reliable, as some glyphs can leak outside of their theoretical box, possibly depending on the used font size, or usage of diacritical marks

God yes. I'm building a UI design tool, and running head-first into this reality. I had no idea it was this hard.

It can go the other way, too. It is mathematically impossible to make a font that has the exact same vertical metrics/line-spacing everywhere, so everyone sets the three value sets provided by the OpenType spec to the same values for web fonts. And then you try to render a font with tall glyphs (i.e. Arabic) on Android and find that it actually looks at the font-wide bounding box in the head table to determine metrics. You can't change them because then Android clips everything legitimately outside that box.