How to increase knowledge on HTML?

Answer 1

what are you trying to learn about HTML. The stuff in sololearn teaches you the fundamentals. I would say, the best way to learn is try it, then ask questions of others if you can't figure it out.
There are also some good tutorials on YouTube and the internet. If you can't figure it out, do a google search on the problem you are havin.

Answer 2

https://developer.mozilla.org/en-US/docs/Web is a good place to start if you like documentation. otherwise, check out codecademy and tutorialspoint