Is it possible to draw a partial circle outline in CSS (open ring shape)?

To create a circle that gradually draws it’s outer path, use SVG.

SVG’s stroke-dasharray property will turn any path into a dashed line, which you can use to your advantage by setting the dash size to be almost as long as the path itself.

Then use a CSS animation to gradually change the stroke-dashoffset to move the dash around the perimeter of your circle.

circle {
  fill: white;
  stroke: black;
  stroke-width: 2;
  stroke-dasharray: 250;
  stroke-dashoffset: 1000;
  animation: rotate 5s linear infinite;
}

@keyframes rotate {
  to {
    stroke-dashoffset: 0;
  }
}

<svg height="100" width="100">
  <circle cx="50" cy="50" r="40" />
</svg>