The box-sizing CSS property defines how the user agent should calculate the total width and height of an element.
Width 300px
🦊content-box
🐻border-box
Add padding 50px
🦊content-box
🐻 border-box
Add border 30px
🦊content-box
🐻 border-box
Add padding 50px + border 30px
🦊content-box
🐻 border-box