Can I use html&css in kivy and python3?
No, unless you are willing to implement html and css parsers and a web renderers, of course. I would say it's better to find why you find it difficult to do certain things with kivy, and how to remedy that. I've yet to find an UI/layout/template that is easier to do with the web technologies than with kivy, but i'm way more profficient at kivy than at web front end, so i might be biased.
I am currently developing an embedded multi-touch kivy program with python 3.52 in linux. Kivy is good, but I find GUI developing is quite difficult in some way, and animations are often quite laggy.
Also, I found that the program gets quite slow if I put many widgets in a single page. My program contains a lot of widgets so I am also thinking implementing a webview could help.
So I am looking for html and css views in particular screens for better look and feel (and maybe improvement of animation by using transition?).
I've found Cefpython, but it says it works only for python 2.7 and says it is not stable. And it seemed like it is just getting url from the internet not bringing html and css from a file system. (Correct me if I am wrong.)
Actually, I don't even need a function as a browser, and all I need is html & css for UI/UX development for couple of screens. 
My embedded system doesn't always support internet(meaning it should work without internet). So I can't put any url in it. Once again, I am only trying to use them for styling. Is there a good way of doing it? Or there is no such thing that I want?
Or is there any other recommendation?