/* <color> values */
border-top-color: red;
border-top-color: #ffbb00;
border-top-color: rgb(255 0 0);
border-top-color: hsl(100deg 50% 25% / 0.75);
border-top-color: currentcolor;
border-top-color: transparent;

/* Global values */
border-top-color: inherit;
border-top-color: initial;
border-top-color: revert;
border-top-color: revert-layer;
border-top-color: unset;

<div class="mybox">
  <p>
    This is a box with a border around it. Note which side of the box is
    <span class="redtext">red</span>.
  </p>
</div>

.mybox {
  border: solid 0.3em gold;
  border-top-color: red;
  width: auto;
}

.redtext {
  color: red;
}