Beginner's mistakes. Rate layout?

Question

code review

Hello.
Please point out any mistakes.
My first two sites:
Simple static - https://github.com/sashami/staticLP
Adaptive + responsive - https://github.com/sashami/adaptiveLP
Now I try to use BEM and preprocessors so as not to pile up the code.
Interested in the opinion of experienced people, what to pay attention to first of all, what mistakes should be eliminated so that they do not become a habit.
Thanks in advance.