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;
}

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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";
}

icon-new-window Download