Looks like it checks for the referrer in main.js and adds the uppercase text-transform if you come from HN:

  let host;
  if (document.referrer) { host = (new URL(document.referrer)).host; }
  if (host === "news.ycombinator.com" || host === "lobste.rs") {
    let style = document.createElement('style');
    // let transform = host === "lobste.rs" ? 
    style.textContent = `
      body { text-transform: uppercase; }
      pre, code { text-transform: none; }
    `;
    document.head.appendChild(style);
    console.log("HN readers clearly can't handle the typing habits of the average trans girl.");
    return;
  }

▲ Centigonal 6 hours ago | parent [-]

Sounds like the author got called out for not capitalizing the start of her sentences[1] and decided that, if HN readers want capital letters, they will get them.

[1] https://news.ycombinator.com/item?id=39027187

	▲	Minor49er 5 hours ago \| parent \| next [-]
		Which is funny because if you engage Reader Mode in the browser, everything becomes proper except sentences, which still start with lowercased letters for some reason. Names are still properly capitalized. It's truly bizarre
	▲	packetslave 6 hours ago \| parent \| prev [-]
		It's a less... dramatic... version of what happens when HN links to JWZ's blog.