Whenever something looks odd on Gwern's site, you can check his A/B testing article[0] to see if it's intentional. In this case, it's BeeLine Reader, his newest experiment[1]. The coloring or shading is supposed to help the eye follow the line as it saccades back and forth.

[0]: http://www.gwern.net/AB%20testing [1]: http://www.gwern.net/AB%20testing#beeline-reader