CSS external links

a[target=”_blank”]:after {
  content: “”;
  width: 16px;
  height: 12px;
  padding-right: 0.5rem; margin
  background: url(/app/uploads/2020/09/icon-new-window.svg) no-repeat right top;
  display: inline-block;
  text-decoration: none;
}

OR 

a[href^=http]:after {
  content: “”;
  width: 16px;
  height: 12px;
  padding-right: 0.5rem;
  background: url(/app/uploads/2020/09/icon-new-window.svg) no-repeat right top;
  display: inline-block;
  text-decoration: none;
}

View Post

Better selector a:not([href*=’yourdomain.com’]):not([href^=’#’]):not([href^=’/’]):after {}

Another refine to external link selection

article.type-post p:not(.specialexception) a:not([href*='yourdomain.com']):not([href^='#']):not([href^='/']):not([href*='mailto']):not([href*='tel']):not([name]):after {
    font-family: 'FontAwesome';
    content: " f08e";
}