Box sizing

Question

Can any one explain this sentence fro me

“Many browsers already use box-sizing:border box; for many form elements but not all which is why text areas look different at width 100%; “

Another question:i saw that box sizing include padding and borders in the width and height
but what about margins how to include them??

Answer 1

border-box: The width and height properties include the padding and border, but not themargin. This is the box model used by IE when the document is in Quirks mode. padding-box : The width and height properties include the padding size, and do not include the border or margin (only in Firefox).
Source: https://daker.me/2013/07/understanding-css3-box-sizing-property.html