That which bractually has nothing to do. What you need is not a line break.
You must specify -webkit-box-orient: vertical;. This is the default value horizontal, so you see this result.
Warning
In fact, you are using an obsolete function that was later replaced by flex.
Cannot be used display: box.
flex W3C, , . .
flex, :
#main{
display: flex;
flex-direction: column-reverse;
}
jsFiddle