Assigning to Parser as I suspect this is what would need to be fixed up to support this.
This patch  demonstrates the large number of rules exist for external links:
The browser matches styles from right to left, so the rightmost selector is really important.
We shouldn't need to have to resort to CSS rules in the form div#content a.external[href ^="https://"] - a link with class external should also have a class describing what type of external link it is if it is necessary. These selectors are less efficient and are not supported by older browsers.
Ideally we should simply these rules to become something like:
background: transparent center right no-repeat;
with rules for each of them for their correct icon.