Responsive Landing Page with vanilla CSS

I found quite difficult overlaying images, it took me a while to have them somewhat similar to the layout that was giving and I feel it could get some improvement, advices are always welcome. The mobile nav was a challenge, and it always is. I added one that I understood because there are so many examples but I don't like to copy things I don't understand. Any advice on improvement will also be well received. What is the best framework for a project like this, that is not Bootstrap? I tried it but fail on customization.