Interesting, how does that add a round trip? For the record here's what I believe to be the common definition of an additional "round trip", in a web development context:
- client requests X
- client gets X, which contains a reference to Y
- therefore client requests Y
So you're starting a new request that depends on the client having received the first one. (although upon closer inspection I think the technique described in the blog post manages to fit everything into the first response, so I'm not sure how relevant this is)
Unless a resource is very small, it won't be transmitted in a single atomic unit. The sender will only send a part of it, wait the client to acknowledge having received them, and only then send more. That requires a network roundtrip. The larger the resource, the more network roundtrips will be required.
- client requests X
- server sends bytes 0-2k of X
- client acknowledges bytes 0-2k of X
- server sends bytes 2k-6k of X
- client acknowledges bytes 2k-6k of X
- server sends bytes 6k-14k of X
- client acknowledges bytes 6k-14k of X
- server sends bytes 14k-30k of X
- client acknowledges bytes 14k-30k of X
- server sends bytes 30k-62k of X
- client acknowledges bytes 30k-62k of X
- server sends bytes 62k-83k of X
- client acknowledges bytes 62k-83k of X
- client has received X, which contains a reference to Y
- therefore client requests Y
It's all about TCP congestion control here. There are dozens of algorithms used to handle it, but in pretty much all cases you want to have some kind of slow buildup in order to avoid completely swamping a slower connection and having all but the first few of your packets getting dropped.
Not just modern. This was even more significant on slow connections, so they've kind of always done that. One could even argue that HTML, HTTP (specifically, chunked encoding) and gzip are all intentionally designed to enable this.
Chunked encoding is required for the server to start sending the document before it has been fully generated (or the size becomes known) but nothing stops the client from parsing not yet complete documents without it.
