How do i create transparent triangle with thin gray borders css

I want to create a triangle that is white but has got thin gray borders. I have tried this code

.arrow-down {
  width: 0;
  height: 0;
  border-top: 20px solid #f00;
  border-right: 20px solid transparent;
  border-left: 20px solid transparent;
}

<div class="arrow-down"></div>

But it gave me triangle filled with red.