Responsive animated age calculator app using HTML, CSS, JS

What are you most proud of, and what would you do differently next time?

I learned a lot about input fields and JavaScript Date object. I learned how to connect three consecutive inputs in such a way that when each one is filled, it focuses on the next one, and when backspacing in each one (if it is empty), it focuses on the previous input. I also add the arrow key functionality to the inputs. When the user presses the right arrow key, it moves the caret cursor to the right, and when the user presses the left arrow key, it moves the caret cursor to the left. I used the following code to achieve this functionality:

What challenges did you encounter, and how did you overcome them?

I had some challenges on using Countup.js library to animate the age numbers but finally I've found the solution, so you can see the numbers animated!