Us there a way to "favourite" a specific lesson, so I can go back to it later and review?

Answer 1

Nope. Not that ive seen. Would be a great feature though

Answer 2

Move on to w3schools. Better references and more in-depth explanations. Don't bother with buying certificates.

And, when you become more accomplished, this:
https://m.reddit.com/r/webdev/comments/2y864m/are_w3schools_web_certificates_worth_it/