Validate data using JS and responsive with css rules

The difficult thing for me was the fact of the animations, I had mistakes in that part that took me time to solve, in addition to the responsive theme that I feel that I did not do it well due to the structure of my html but it is something that remains an experience for me

The area of my code that I am not sure about is the responsive part since I do not have the necessary knowledge to make a better design thinking about the user experience

I would like to know the opinion of the community of: What are the best ways to make a page responsive?