Simple calculator webapp, using JavaScript, CSS and HTML

This is my first website project but please be as harsh as you can! You may notice that I chose to do specific things the hard way but that is simply because I wanted to have as much variety as possible with my code, for example I could have simply used a class for the css of the reset and DEL button but I wanted to get an idea of how nth-child works. My calculator algorithm may be working exactly as I want it to however I feel like I made it harder than it needs to be, and any tips on how to improve the algorithm whilst having the same results would be great.