Lead Developer
This most likely has to do with the border attributes you’ve added.
When you add a border to an element which has a width, the border adds to that width.
A better idea is to apply the border to the element inside the element with the width.
For example, instead of applying a border to .main-navigation, apply it to .inside-navigation.
The same principal applies to padding and margin.
Play around with that and let me know if it fixes it 🙂