I think this could be improved by moving the stylesheet up in the DOM (in the head, and not at the end of the body as it's the case now).
That's ... unusual to say the least and if I'm honest, the wording "most stupid, brain dead thing I've seen in a long time" was the first thing that crossed my mind.
That's quite an ugly optimization step. You left out the step where you inline the necessary CSS styles in the head that's why the unstyled page flashes up for a second. But as you use bootstrap, this might be a lot to inline and not worth the effort.
I'd say this is a case where you shouldn't care too much about a highscore :-)
Maybe I'll just bite the bullet and do my own CSS.
Framing it as an absolute where you shouldn't use two tools that make it easier to make a website is unnecessary and makes you look dogmatic rather than wise.
The author obviously had plenty of page weight to spare.
BUT, I wanted to have a play with Bootstrap, which I hadn't used before and it was/is comforting to have the ability to expand the look to include other features if I need to :)
(IIRC the 1st edn AD&D DM's guide dates to roughly 1982. (Can't find my copy to verify: that's going from memory of when I got mine.))