> Also: What did people screw up with HTML in your opinion?

The problem with PDFs is that you need to create multiple layouts to make them look good in print and on a variety of commonly used screen sizes; all those layouts is extra work. HTML, by its very nature, doesn't have this problem, and yet somehow today we still have to design multiple layouts to support print and common screen sizes. And in practice, we usually don't - instead, we design one layout optimized for mobile phones, and ignore how bad lit looks on desktop or in print. "Responsive web design" turned into forcing HTML to behave like a PDF, except using "iPhone" instead of "A4" as the size.

If you make your PDFs in A5, you can print two of them on an A4 paper and read the paper in landscape orientation. For the same reasons the size fits well for displaying on a computer screen and on a tablet/e-reader. It's still a bit too big to squeeze down to a cell phone, but at least better than A4/Letter size.

As for responsive HTML, it's the responsibility of the designer to make it work if he/she is worth their salt. Like you say, HTML without CSS is already responsive. If businesses understood that there are a big segment of customers who will always use their computer and never their phone when it's time to make a purchase, perhaps they'd be better at it.