Couldn't even that be abused? A link that's just a square character styled either black or white could look really similar to the demo.

Why can't we have situation where the CSS tells the browser what colours things should be etc, the browser does it, but the site cannot retrieve information about what colour the links are?

There's all kinds of things browsers do that no amount of coding will help me access because there's no public API for it. Why should this be any different?

This is the whole point of link, even if the browser doesn't returns the correct information, there's still some ways to leak it through user action.

Agreed. Browsers should stop allowing a lot of things. In my opinion, they give way too much freedom to websites. That freedom can and should be taken away if it's abused.