Responsive Clipboard Landing page with pure CSS

As it was just a simple landing page it wasn't much difficult to create but I had my hiccups.

Taking a look at my code I still wonder whether I took a good approach by designing mobile first and then making it responsive for the desktop version?

Another thing I am unsure of is the structure I chose for "Keep track of your snippets" section as the desktop and mobile version were quite different in layout perspective. I wonder if I chose a opted for a better approach or not?