Answer 1

HTML, CSS and a little Javascript. Basic knowledge based questions are mostly seen in the challenges. The math problems are not in the HTML challenges unlike the other languages, that's why after a certain time people can memorize the answers. The quizzes are quite easy and repetitive, so people get 5 correct answers continuously.
Hope it helped. Good luck!👍🙃🍫💕⚽🌹

Answer 2

None. Just select HTML when you want to challenge someone.

Answer 3

Well, if I recall correctly, challenge questions are by users as well, so they might not necessarily be completely course based. There are other avenues of learning outside of SL that could've affected the questions submitted.

Answer 4

HTML, in case you want to become a web developer then you can learn CSS & Javascript including its frameworks. Well you can choose which framework is for you..