Hi Toh!

First, I thank you very much for posting and sharing this good piece
of code!

I like it because, it's simple, it's embedable, no dependency, pure
javascript.

I found your code search for a simple quiz code. I consider quizzes as
a fun learning learning experience on something.

Some things I'd like to improve on it:

ux/reload) The user has to reload the page to restart the quiz and 
it's not evident. How about making the last button a possibility
to redo the quiz from start and make a better score?

ux/where I am) The user doesn't know how many questions there are in
the quiz, where he is along the quiz, how good he is. Knowing where we
are in a serie makes it easier to reach the end.  This one is easy to
add, as I could do it.

css functional) The quiz.css is not so cosmetic: if you comment it in the
html, then the quiz functionally doesn't work the same; we have to
click twice on a button to go to the next question, and the second
click has to be outside the round of the radio button, on the label.
I don't know how to fix that but it's not important anyway, the css is
needed for cosmetics.

css cosmetic) this quiz code is to be embedded in a page then I suggest
that the cosmetics are inheritated. 
I'm also adept of minimal css, the less to achieve a goal, the better.

dev testing) I don't know how do you test it while coding and 
modifying it. A manual loop browser_reload_run_editor?

dev jslint) I paste it to jslint (https://www.jslint.com/)
and it didn't pass the lint test.

dev data load) How about putting all the Q/R in a dedicated text file
to maintain a single quiz.js and quiz.css and have multiple quizzes
using the same functional code?