HTML5 or not, CSS rules don't typically need to be duplicated across multiple stylesheets, except when using Media Queries.
Keep in mind that something written in an external stylesheet can be overwritten in another or by styles within the page though.
Ok, think I understand; but I'm specifically asking about that particular display rule for html5, should I put that in my internal style or my external style sheet?
I'm sorry, the point I was trying to make was it honestly doesn't matter "where" you put it (external or embedded), as long as it's there and isn't being overwritten by css from another source.
Which stylesheet do you use for the actual layout of the page?
That's where I would put it. Whether you want it there is up to you, it could be in either, but it's not needed in both. Your code will be lighter if you put it in the external file, since you would use the same file on every page, seems a waste to have to paste it into every page in the head section.
I usually put all of my css in one external file. Unless that file gets huge, then I will separate out layout from content style and have two external files.
There is literally no reason I can find to have your css embeeded in the <head> section of a page.