Jeremy takes a fresh look at this here in 2018, noting that the first published desire for this was 15 years ago. All the same use cases still exist now, but perhaps slightly more, since web typography has come along way since then. Our desire to do more (and hacks to make it happen) are all the greater.
I seem to recall the main reason we don't have these things isn't necessarily the expected stuff like layout paradoxes, but rather the different typed languages of the world. As in, there are languages in which single characters are words and text starts in different places and runs in different directions. The meaning of "first" and "line" might get nebulous in a way specs don't like.