Browsers are able to parse a webpage from 1996. I don't know what the argument in the linked comment is about, but in this one, we discuss the relevance of creating a 1996 page vs a pelican on a a bicycle in SVG.
Here is Gemini when asked how to build a webpage from 1996. Seems pretty correct. In general I dislike grand statements that are difficult to back up. In your case, if models have only a cursory knowledge of something (what does this mean in the context of LLMs anyway), what exactly they were trained on etc.
The shortened Gemini answer, the detailed version you can ask for yourself:
Layout via Tables: Without modern CSS, layouts were created using complex, nested HTML tables and invisible "spacer GIFs" to control white space.
Framesets: Windows were often split into independent sections (like a static sidebar and a scrolling content window) using Frames.
Inline Styling: Formatting was not centralized; fonts and colors were hard-coded individually on every element using the <font> tag.
Low-Bandwidth Design: Visuals relied on tiny tiled background images, animated GIFs, and the limited "Web Safe" color palette.
CGI & Java: Backend processing was handled by Perl/CGI scripts, while advanced interactivity used slow-loading Java Applets.
I may have missed something but where are we saying the website should be recreated with 1996 tech or specs? The model is free to use any modern CSS, there is no technical limitations. So yes I genuinely think it is a good generalization test, because it is indeed not in the training set, and yet it is easy an easy task for a human developer.
I'd be curious about that actually, feel like W3C specifications (I don't mean browser support of them) rarely deprecate and precisely try to keep the Web running.