It seems to work well, with good performance overall, but it indeed breaks some websites. That why it needs a exceptions list feature (if possible with smart segments of url, like: whitelisting .example.com' , 'example.com/subcategorypages' or "subdomainonly.example.com' ), and maybe for later, alternative methods to darken the pages (simple inverse filter, CSS) if the main one doesn't work as expected.
Also, it'd be cool to let the browser highlight with a different color visited links.

Encouragement to the dev!