Edit in JSFiddle

<p>
   An <a href="https://example.com" target="_blank">external link<span class="visually-hidden">(opens in new window)</span></a> that goes to another site
</p>
<p>
   An <a href="https://yoursite.com">internal link</a> that stays in the same window
</p>

a[target="_blank"]::after {
  content: "";
  width: 1em;
  height: 1em;
  margin: 0 0.05em 0 0.1em;
  background: url(data:image/svg+xml;base64,PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHZpZXdCb3g9IjAgMCAxNiAxNiIgd2lkdGg9IjE2IiBoZWlnaHQ9IjE2Ij48cGF0aCBkPSJNOSAyTDkgMyAxMi4zIDMgNiA5LjMgNi43IDEwIDEzIDMuNyAxMyA3IDE0IDcgMTQgMlpNNCA0QzIuOSA0IDIgNC45IDIgNkwyIDEyQzIgMTMuMSAyLjkgMTQgNCAxNEwxMCAxNEMxMS4xIDE0IDEyIDEzLjEgMTIgMTJMMTIgNyAxMSA4IDExIDEyQzExIDEyLjYgMTAuNiAxMyAxMCAxM0w0IDEzQzMuNCAxMyAzIDEyLjYgMyAxMkwzIDZDMyA1LjQgMy40IDUgNCA1TDggNSA5IDRaIi8+PC9zdmc+) no-repeat;
  background-size: contain;
  display: inline-block;
  vertical-align: text-bottom;
}

.visually-hidden { 
    border: 0;
    padding: 0;
    margin: 0;
    position: absolute !important;
    height: 1px; 
    width: 1px;
    overflow: hidden;
    clip: rect(1px 1px 1px 1px); /* IE6, IE7 - a 0 height clip, off to the bottom right of the visible 1px box */
    clip: rect(1px, 1px, 1px, 1px); /*maybe deprecated but we need to support legacy browsers */
    clip-path: inset(50%); /*modern browsers, clip-path works inwards from each corner*/
    white-space: nowrap; /* added line to stop words getting smushed together (as they go onto seperate lines and some screen readers do not understand line feeds as a space */
}