Hình minh họa : 

(Xem đầy đủ tại https://dnniscode.blogspot.com/)
<div class='cursor'>
  <div class='inner-cursor'/>
</div>
<style>
.cursor {
  position: fixed;
  width: 30px;
  height: 30px;
  border-radius: 50%;
  background-color: transparent;
  border: 2px solid #d1cfcf;
  pointer-events: none;
  z-index: 9999;
}
.inner-cursor {
  position: fixed;
  width: 10px;
  height: 10px;
  top : 100%;
  left : 100%;
  transform: translate(100%, 100%);
  border-radius: 50%;
  background-color: transparent;
  border: 2px solid #d1cfcf;
  pointer-events: none;
  z-index: 9999;
  transition: all 0.01s ease-in-out;
}
</style><script>
document.addEventListener("mousemove", function(event) {
  const cursor = document.querySelector(".cursor");
  const innerCursor = document.querySelector(".inner-cursor");
  
  cursor.style.left = event.clientX + "px";
  cursor.style.top = event.clientY + "px";
  
  innerCursor.style.left = event.clientX + "px";
  innerCursor.style.top = event.clientY + "px";
});
 </script>