Is Sololearn enough to learn complete HTML ?

Answer 1

it will teach you a lot of basic principles and from those you can learn on. do a search on this forum and you will find many websites that can help you on further after the course (mostly free).

Answer 2

no, but it does help you along. you need to practise and work on it, use it in real projects.
But it will give you a proper basis to learn on from.
Do code challenges from the learn section, those are great for continued learning.