QR code component made with HTML(5), Flexbox, Sass and Mobile first

What I learned and the things I should keep in mind can be found in the README.md file on my GitHub repository.

A couple of questions:

1. Are there any remarks on how I implemented HTML(5) in my index.html?
   Like, would you do it in another way, and why?

2. Are the comments I defined within my stylesheet clear, or is there a different way I should use them?

3. What about my written CSS in general. Are there any remarks on that?

I also created an alternative solution to the raster image that has been used for the QR code (supplied by Frontend Mentor). This alternative solution can be found in the README.md in my GitHub repository.