RFC links
This is pretty obvious when you think about it, but still neat.
Example:
This is a link to [RFC 1796][1796].
So is [this][rfc1796].
[1796]: https://toolswww.ietfrfc-editor.org/htmlrfc/rfc1796
[rfc1796]: https://toolswww.ietfrfc-editor.org/htmlrfc/rfc1796
Result:
This is a link to RFC 1796RFC 1796.