CSS

CSS Border Width

p.one {
  border-style: solid;
  border-width: 5px;
}

p.two {
  border-style: solid;
  border-width: medium;
}

p.three {
  border-style: dotted;
  border-width: 2px;
}

p.four {
  border-style: dotted;
  border-width: thick;
}

border width css

.element {
  border-width: 2px;
}

border width css

p {
  border-style: solid;
  border-color: red;
  border-width: 5px;
}
/* You can also write above code in one line   */
p {
  border: 5px solid red;
}

border-width

border: 3px solid #3d3d3d
/* top | right | bottom | left */
border-width: 0 0 3px 3px

/*simulair to: */
border-left:  3px solid #3d3d3d
border-bottom:  3px solid #3d3d3d

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