In the absence of someone giving you a better answer, my hazy memory says 2015 or early 2016?
Given what we know about Apple's hardware platform direction, on hindsight, it seemed like a good strategy. Note: I am talking about the team merging, not merging macOS and iOS themselves.
No because that additional CSS file can only be fetched after the initial HTML file has been fetched, not simultaneously; thus requiring another roundtrip to the server. (HTTP/2 Server Push promised otherwise but it was never implemented correctly.)
Latency being much more constraining than bandwidth, embedding a stylesheet that’s small is nearly always the correct tradeoff.
I'd look to what Wikipedia provides as a document reading experience. The problem with nav is that there weren't enough tags so developers will rightly use them for other things, such as breadcrumbs. I'm looking for exclusive ToC that will allow reader mode to go further.
