I'm not aware of any tools; I converted this document by hand in a few hours. GIF is a popular standard with cultural value, which is why I would do this; I wouldn't do it for any random obscure document. The IETF RFC website has links to HTML and PDF versions, but...

For most standards documents, the HTML and PDF versions look just like the plain text version, adding little or no value (e.g. https://www.rfc-editor.org/rfc/rfc1321.txt ; https://www.rfc-editor.org/rfc/rfc1321.html ; https://www.rfc-editor.org/rfc/pdfrfc/rfc1321.txt.pdf ).

The only standard I am aware of with a decently formatted version is DEFLATE ( https://www.rfc-editor.org/rfc/rfc1951.txt ; https://www.rfc-editor.org/rfc/rfc1951.pdf ).