That's not an uncommon goal but one that is usually unrealistic until after the designer takes a realistic look at the numbers of each version of each device in the wild and decides which ones are "musts" and which ones are "nice to have". Coding for "all" devices is expensive and rarely worth the cost (impractical vs impossible). I hear "code for all browsers" quite frequently, but during subsequent discussion I've not met anyone who insisted on coding for Netscape.
You sound like you're well read on the subject. But "just in case" you missed "can i use", check this out. I'm not taking sides, just throwing out another reference source. nth-child() and the plus (adjacent sib selector) each boast 98%+ cross browser compatibility so it seems like a tossup to me.
As always, exceptions are handled on a case by case basis.