css box sizing
box-sizing: border-box;
box-sizing: content-box;
/*content-box gives you the default CSS box-sizing behavior. If you set an element's
width to 100 pixels, then the element's content box will be 100 pixels wide, and
the width of any border or padding will be added to the final rendered width, making
the element wider than 100px.
border-box tells the browser to account for any border and padding
in the values you specify for an element's width and height. */