When I rebuilt this site earlier, one page failed validation. It was easy to see why, since the error message was
Bad value footnote for attribute rel on element a: The string footnote is not a registered keyword. Someone
had reported it last year but no-one had fixed it.
Seemed straightforward, so I pushed a fix which works for this site. If you're here because you searched for the above error message, an update of blackfriday markdown should sort it out now. Looks like other markdown → html generators have the same issue - I don't use them, but the fix should be just as easy.
When will it be fixed? Sooner, with your help.