Sololearn is a website and app for learning to program.
If my explaining is hard to understand then I recommend you to run the code on your dev platform or on a debugger like pythontutor. Remember, I'm just a student.
Let me know if you find any typo error, english isn't my mother language.
C certificate.
HTML certificate.
CSS certificate.
JavaScript certificate.
GO certificate.
JS Gamedev certificate.