Basic CSS Boilerplate with Reset and Border-Box
Copy and paste this CSS (line 1 - 15) to the top of your own CSS (if you didn't already).
In contains the "CSS Reset" to eliminate the browser default styling as well as the "Border-Box" sizing that simplifies size calculations (the border width and paddings are being ignored).
See the Pen basic boilerplate by Oliver Roeger (@uic-des) on CodePen.