Answers for "width of border css"

border width css

.element {
  border-width: 2px;
}

border width css

element {
	border-width: 10px;
  	/* Or use shorthand border property */
  	/* width style colour */
  	border: 10px solid black;
}

border width css

#some-div {
  /* [WIDTH] [FILL MODE] [COLOR] */
  border: 5px solid #f00; 
  
  /* draws solid a red line of 5px
  between the padding (inside the div)
  and the margin (outside the div) */
}

border-width in percentage

You can not set Border Width in the Percentage its the rule 
But you can able to do this just by wrapping into a new div 
and in most of the cases you did not need that