You can use the a:link selector to only set the color of regular links that have not been visited yet. Visited links will keep their default color (purple or whatever color chosen by the browser or the user).
So: don't use a { color: ... }, use a:link { color: ... }
If you don't set it, you can't guarantee that a:visited contrasts sufficiently with a:link or the background.
Absolutely.
Oh the irony. He himself has failed to include a:visited in his CSS stylesheet.
No I believe he did, look at the pingbacks!
Also "Confession: I added this rule minutes before I posted this article"
override it globally in your browser with userstyles!