Answers for "default box-sizing"

css box-sizing

/* Always keep this code inside your css file*/
*{
  box-sizing: border-box;
}
.div-1{
  /* width: 100%; Default - Block Element*/
  padding: 10px;
  box-sizing: border-box; 
  /* 
    content-box is default for box-sizing
    content-box will remove border-box effect.
  */
}
/*
  after applying padding [border also will increase the width]
  10px added to left and right from padding
  the .div-1 width is now 100% + 20px, and that may cause errors in your layout
  according to your work.
  to solve the problem and force the width to be as i give to it
  we use box-sizing => our div width will not be affected by padding or border
*/

box sizing border box

*{
 box-sizing: border-box;
 }

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. */