Externally linked sources introduce a backward-compatibility problem of their own. E.g, I recall early versions of IE only supporting inline JS. So, if you want to support as much as possible as far back as possible, inlining is the way to go.
(Since any extensions had this problem, when they were introduced, they all support enclosing HTML comments. Otherwise, there wouldn't have been any chance for becoming broadly adopted.)
Regarding dithering: I recall a few sites with duotone dithering, which was a nice effect. And, of course, to bring down file sizes and optimize results, you could always manually compose from dithered and undithered images using the same palette.
(Since any extensions had this problem, when they were introduced, they all support enclosing HTML comments. Otherwise, there wouldn't have been any chance for becoming broadly adopted.)
Regarding dithering: I recall a few sites with duotone dithering, which was a nice effect. And, of course, to bring down file sizes and optimize results, you could always manually compose from dithered and undithered images using the same palette.