I honestly never understood this, but I believe it has something to do with margin: 0 auto;
I typically set containers with min-width: ; for the header, content, and footer.
I believe this fixes it:
Code:
.line { min-width: 50em; }
I believe it is because the nav with width of 50em is setting the 100% width of the parent, when we say min-width, we are allowing it to go past the set 100%, along with margin: 0 auto;
I remember reading about this a long time ago, and now that I can fix it, I don't remember why
Don't worry about it, it's something you learn how to fix in time