CSS suggestions

[LaLeonie]

Just a couple of suggestions to polish up your CSS:

When you declare a font-family, it's best practice to provide an alternative font and a serif/sans-serif flag the browser can fall back to, like font-family: Arial, Helvetica, sans-serif;

Using border-box can make your life easier later on, so I'd suggest to add this code to the start of your CSS:
html{
box-sizing: border-box;
}

*, *:before, *:after{
box-sizing: inherit;
}