Can section and article elements be nested to each other?

Answer 1

Yes, in a reasonnable mesure of content logical... you can define <section>s in an <article>, or reversly, you can have <section>s into <section>, but to many level of nested ones is probably a bad design structure ( as there's always particular cases ^^ ), and <article> inside <article> don't have really meanings :P

Answer 2

https://code.sololearn.com/WG5ferTgdbA6/?ref=app

Answer 3

yes, but at a regulated proportion, so browsers could understand your web structure in HTML5 format.