Why is it awful? Most barebones websites are naturally responsive..

I don't usually see this because it seems to require intentional design to work on mobile. The original post has an example that doesn't lay out well on mobile, or just a very tall and thin desktop window.

Talking about the url provided in the OP, one click on Firefox for mobile and it should be obvious. Text wider than then the screen, yellow text on white background. line spacing that's too tight, a background image that obscures text...