Legibility and usability. Like many old sites this one uses small fonts, small elements, and has tiny targets. Especially when working on a large display. I make liberal use of Firefox's zoom feature to compensate, but better defaults don't hurt.

And yes it works on a phone as you can easily zoom in, but inclusion of responsive design, e.g. moving the sidebar to a footer, and use larger fonts would significantly improve legibility and usability. This is not incompatible with a simple and fast-loading site.