HTML5 and CSS3 are just HTML and CSS, so there's no problem regarding support in code editors. You just type your code as usual. Code coloring (if you have that) will continue as normal. Is there something else you're looking for?
I already got used to Notepad++, and it's a good thing that it supports HTML5 & CSS3 as I start my journey into learning HTML & CSS.
Cool, yes, I understand, and Notepadd++ is very good. Remember that HTML5 and CSS3 are mostly experimental at this stage, so get your grounding in current (X)HTML and CSS first, as these are the important things at this stage. Then take a look at what extra tid bits the new versions bring to the table. They just add some new features, rather than representing something new.
I wound rate this tool highest. I have used textpad, edit++, but notepad++ is light weight with heavy functionality if use all of them. The interface is great. I don't know why some of the web development farms still using years back old textpad, may be not aware of new cool things.