Box model and Block formatting context.
box-sizing: boder-box; (CSS box model of padding and border not included in the width)
box-sizing: content-box; (CSS box model where padding and border are included in width)
Initially IE got it right and then people detested it and now we again are getting to it :)
Block formatting context: https://www.youtube.com/watch?v=8YtQwv1cUVs