Mobile-first solution using HTML5, CSS, Flexbox, JS (+ ES6), Gulp etc.

In the README.md file (on my GitHub repository), you can find information such as:

1. Which languages and tools I used to build the challenge
2. What I learned
3. Things I should keep in mind for the next challenge
4. My continued development

However, there are some questions I have, which you can find below:

1. Are there any remarks about how I implemented the HTML5 markup? Like did I use <header>, <article>, <section> etc. in the right way?
2. Should I have been using a <h1> in both cases or <h2>? (In this case I used both a <h2>, because I know that you are only allowed to use a <h1> once, and found it quite odd to use a <h1>, and for the thank you card a <h2>)?
3. Are there any remarks on the way I used whitespace in both HTML and JavaScript?
4. What about my comments? Did I describe them clearly? Or did I use too much of them?
5. The boolean condition I used on line 97 (index.html), is this clear enough? Or should I have been doing this maybe in another (better) way? Like, how would you have written this condition?
6. Do you have any other tips or things for me which I should keep in mind?

And at last, thank you for taking your time to check the solution of my challenge. I really appreciate it!