2021 Solution
a[href]:not(:where(
/* exclude hash only links */
[href^="#"],
/* exclude relative but not double slash only links */
[href^="https://stackoverflow.com/"]:not([href^="//"]),
/* domains to exclude */
[href*="//stackoverflow.com"],
/* subdomains to exclude */
[href*="//meta.stackoverflow.com"],
)):after {
content: '↗️';
}<strong>Internal sites:</strong>
<br>Lorem <a href="http://stackoverflow.com">http://stackoverflow.com</a> ipsum
<br>Lorem <a href="https://stackoverflow.com/a/5379820">/a/5379820</a> ipsum
<br>Lorem <a href="https://stackoverflow.com/a/5379820">//stackoverflow.com/a/5379820</a> ipsum
<br>Lorem <a href="http://stackoverflow.com/a/5379820">http://stackoverflow.com/a/5379820</a> ipsum
<br>Lorem <a href="https://stackoverflow.com/a/5379820">https://stackoverflow.com/a/5379820</a> ipsum
<br>Lorem <a href="https://meta.stackoverflow.com/">https://meta.stackoverflow.com/</a> ipsum
<br>Lorem <a href="ftp://stackoverflow.com">ftp://stackoverflow.com</a> ipsum
<br><br>
<strong>External sites:</strong>
<br>Lorem <a href="ftp://google.com">ftp://google.com</a> ipsum
<br>Lorem <a href="https://google.com">https://google.com</a> ipsum
<br>Lorem <a href="http://google.com">http://google.com</a> ipsum
<br>Lorem <a href="https://www.google.com/search?q=stackoverflow">https://www.google.com/search?q=stackoverflow</a>
<br>Lorem <a href="https://www.google.com/search?q=stackoverflow">//www.google.com/search?q=stackoverflow</a>
<br><br>
<strong>Other anchor types</strong>
<br>Lorem <a>no-href</a> ipsum
<br>Lorem <a href="#hash">#hash</a> ipsum2014 Solution
Using some special CSS syntax you can easily do this. Here is one way that should work for both the HTTP and HTTPS protocols:
a[href^="http://"]:not([href*="stackoverflow.com"]):after,
a[href^="https://"]:not([href*="stackoverflow.com"]):after {
content: '↗️';
}