Box Sizing
C O N T E N T B O X C O N T E N T B O X C O N T E N T B O X C O N T E N T B O X C O N T E N T B O X
← 150px total →
total = width + padding + border
5px + 20px + 100px + 20px + 5px
B O R D E R B O X B O R D E R B O X
← 100px total →
total = width = padding + border + remainder
100px = 5px + 20px + remainder + 20px + 5px