Earlier quoted context omitted.
Why are you generating styling at all? It's not dynamic information. It's a static asset that can be cached. Semantically correct markup uses fewer tags, so the output is smaller (also, there'd be no online styling, which would also make the page smaller). Even a small difference on a popular site could matter. But only you can answer that for HN. Look, do what you want. You want to write bullshit markup, be my guest…
Generating and caching are orthogonal questions. You can generate stuff, and then cache it. And in fact HN does do a lot of caching. I wasn't asking how much faster "semantically correct markup" would make HN pages render as a rhetorical question. I'm genuinely curious. If you want to make claims that x is faster than y, you should be prepared to back them up with numbers, not merely with more heated language.
To me, it sounds like the caching you're talking about is on the server side. I think you mean something similar to memoization, so you can avoid doing some expensive query or calculation. That is worth doing, but it's still possible to organize the output of those caches in an inefficient manner, and incur unnecessary network overhead. The "semantically correct" part of the markup being advocated here isn't that interesting, if you ask me. What is interesting is the claim is that you can generate less markup per page, and get the same display.
The balance between the repeat visitor cache behavior and the initial number of HTTP requests and latency for a first-time visitor can get hard to judge. Without lots of time to measure the various alternatives and mitigation tactics, it's best to try and generates as little markup as possible. That's where so-called "semantic markup" comes in. Usually it's just less markup, and does better.
Another thing to take into account is the layout behavior triggered in various browsers by the markup you're generating. HN is pretty simple and should render instantly, but it doesn't in anything I've tested (Firefox, Chrome, Safari). Each of them redraw the scrollbar one or more times. That could be due to the tables.