Recently asked Codex (GPT-5.2) to write a small single-page HTML frontend to debug some REST endpoints. As it was just a one-off tool, I put in no instructions about looks or styling at all. Lo and behold, the tool it wrote came with exactly that round-box style.

It seems to be the "default" style of some models for some reason.

Which makes me wonder if people already experimented with different style suggestions to get different results: "Make it look like an 1998 GeoCities page" / 2005 Facebook / Newgrounds / DeviantArt / HN / one of those Windows XP simulators with built-in window manager / etc

I vibe code web apps with Google's Gemini and I think it actually mimics Google's UI and UX because I see similarities between my vibe coded web apps and Google's web apps.