Wouldn't highlight previously viewed videos in the "related videos" window for me.
Me gusta mucho este script.
Trabaja muy bien y si eres un asiduo de youtube te vendrá como anillo al dedo.
Fixes one of my biggest gripes about YouTube. It is so nice to finally be able to avoid (or find) vids I've already seen.
...I've commented this line in your script
aXpath = '//a[contains(@href, "&list")]'; // playlists
because it breaks links in results page (when filtered by playlist) and in watch page (when viewing a playlist)
Hope you keep your script updated!
Awesome. Keep it up.