I'm not asking for selector mutation observers. This is nice to have, but a library works just fine. I'm suggesting a performance optimization so that libraries can effectively observe the entire page DOM, without creating yet another mutation observer for the entire DOM. This can only be done on the browser level, not as a library, since otherwise all libraries would need to agree to use the same dependency, which as you can imagine, is quite unrealistic.
Syntax-wise, I guess it could be something like
document.onAppend(selector, callback);, name TBB. Or just an event on
document, since it wouldn't have anywhere to bubble anyway? Though that sounds dangerously close to mutation events, which had serious perf issues.